Patent Application
20240408815
The present invention relates to a photoresist composition, a system, in particular for 3D-printing, comprising a photoresist composition, a method for producing a three-dimensional structure, and a use of a photoresist composition in 3D-printing.
In her PhD thesis, Marie Göppert-Mayer brought two-photon absorption to light. In a two-photon absorption process illustrated in
Today, two-photon absorption processes are crucial in multiphoton optical microscopy and multiphoton optical lithography, both of which have been commercially available for many years. Multiphoton optical lithography has become a well-established method to fabricate three-dimensional (3D) structures from the nano- to the microscale. In 3D optical lithography aka direct laser writing or 3D laser nanoprinting, two-photon absorption leads to a scaling of the photoinitiator transition rate and hence of the exposure dose proportional to I2, i.e., the light intensity squared. Critically, this quadratic nonlinearity suppresses unavoidable lateral and axial tails of the diffraction-limited laser focus and thereby warrants the crucial concentration of the excitation and the following chemical reaction along all three spatial directions. Importantly, one-photon absorption alone without additional nonlinearities fundamentally cannot provide this concentration in the making of arbitrary 3D structures. To obtain efficient two-photon absorption, mode-locked pico- or femtosecond laser sources are routinely used. Upon diffraction-limited focusing, peak intensities in the range of I=1012 W/cm2 and the corresponding photon irradiance of 3·1031 s−1 cm−2 (at a wavelength of 800 nm) provide appreciably large effective cross sections in the range of 10−19 −10−20 cm2, depending on the photoinitiator molecule.
While being a well-established technology, using femtosecond lasers to obtain efficient two-photon absorption in 3D laser nanoprinting has a number of conceptual and practical drawbacks.
First, when increasing the laser power from the point of sufficient polymer crosslinking upwards, micro-explosions occur due to the onset of three- and four-photon absorption processes and beyond, leading to the unwanted population of high-energy electron states. Typically, the laser power at which micro-explosions occur is less than an order of magnitude above the writing point. Sometimes, the process window is even as small as 10%. Small pollutions or dirt microparticles in the photoresist can trigger micro-explosions even at the writing point. Such events render entire time-consuming 3D-printing jobs useless.
Second, the required femtosecond laser oscillators are still extremely costly.
Third, the femtosecond laser and its supply take up a considerable volume fraction of the overall instrument. This combination of cost and size has so far prevented 3D laser nanoprinters to become anywhere close to as ubiquitous as graphical 2D laser printers or other forms of optics-based 3D-printers.
Thus, a problem to be solved by the present invention is to improve 3D-printing.
This problem is solved by the independent claims. Preferred embodiments result from the respective dependent claims.
An aspect of the present invention relates to a photoresist composition, comprising:
Advantageously, the above-mentioned photoresist composition contributes to miniaturization of 3D-microprinting and 3D-nanoprinting devices, herein also referred to as 3D-printing devices, and renders 3D-printing more cost-efficient. Moreover, the photoresist composition provides a comparable transition rate scaling regarding the intensity of light I from the ground state to the polymerization-inducing state of IN with N>1, where N=2 as in the case of two-photon absorption-based 3D-printing, or, more generally speaking, where N≥2 as in the case of multiphoton absorption-based 3D-printing. As a result, the photoresist composition enables more efficient 3D-printing while maintaining high printing resolution. Using the above-mentioned photoresist composition in 3D-printing, three-dimensional micro- and/or nanostructures may be printed, being extendable to large scale structures.
All the drawbacks of state-of-the-art 3D-printing as mentioned above may be prevented.
According to the present invention, the term “polymerizable monomer” is not particularly restricted and may include one type of polymerizable monomer or may include a mixture of two or more polymerizable monomers. Also, the type of polymerization monomer is not limited and may, for example, include monomers suitable for radical polymerization, cationic or anionic polymerization, as well as monomers following specific polymerization mechanisms such as thiol-ene-based or thiol-yne-based polymerization, or Diels-Alder cycloaddition.
Examples of polymerizable monomers include acrylates, methacrylates, acrylamides, methacrylamides, styrenes and other vinyl compounds, alkenes including dienes, alkynes, lactones, lactams, (cyclic) amines, and thiols.
The expression “photoinitiator” used herein is not particularly restricted and includes such compounds which create reactive species such as radicals, cations or anions upon irradiation with light, for example of the UV, VIS or IR wavelength range. These reactive species are then capable of initiating and/or promoting polymerization of the polymerizable monomer.
According to the present invention, the term “polymerization inhibitor” is not particularly restricted and used in a general sense to describe any compound capable of preventing unwanted polymerization. As such, the term “polymerization inhibitor” as used herein includes both retarders and true inhibitors. This may also include singlet or triplet state quenchers.
In quantum physics or quantum chemistry, electronic quantum mechanical energy states or quantum states normally refer to a set of quantum numbers, such as energy level number and spin, which describe the dynamics of electrons in a quantum system such as an atom or a molecule. Quantum states mathematically refer to wavefunctions which are determined as solutions of the Schrodinger equation of an atom or a molecule. The electronic quantum states may, in the framework of this description, be single-particle states or wavefunctions of the electrons of an atom or a molecule. These single-particle states may be solutions of the Schrödinger equation for the electrons of an atom or a molecule using a mean-field approximation of the electrostatic interactions of the electrons as, for example, in the so-called Hartree-Fock approach or density functional theory. The single-particle states of the electrons obtained in such a way may be atomic orbitals or molecular orbitals.
In this respect, the term “ground state” refers to the energetically highest single-particle state, i.e., atomic or molecular orbital, that an electron of the photoinitiator may have or—in quantum chemistry language—occupy below or at room temperature. The term “intermediate state” refers to an orbital which is higher in energy than the ground state. The term “polymerization-inducing state” refers to an orbital higher in energy than the intermediate state, which when occupied by an electron by excitation of the electron from the ground state or intermediate state induces the polymerization of the polymerizable monomers in the photoresist composition. By occupying the polymerization-inducing state or orbital by exciting an electron, a chemical reaction resulting in the polymerization of the polymerizable monomers of the photoresist composition is activated or sets in.
In this description, the term “optically excitable state” refers to an electronic quantum state or orbital which may be reached or occupied by light excitations, i.e., by electron-photon interactions, such as electron-photon scattering processes. An electron may be excited by being hit by a photon and/or absorbing the photon of a specific wavelength and/or frequency corresponding to the excitation energy from the previous state to the optically excitable state.
The term “substantially optically excitable state” refers to the situation that a state to be excited may be occupied only by an optical excitation by an electron-photon hit and/or exciting an electron by absorbing a photon, or the state to be excited may be occupied by an optical excitation of an electron to a metastable state from which the electron may then relax to the state to be excited.
In this description, the term “lifetime” means the time until an electron relaxes from a state to an energetically lower state or the time after which a state is de-occupied by an electron. The relaxation process or de-occupying process may be a radiation-free process or may occur by radiation. That is, an electron may emit a photon of an energy corresponding to the energy difference between the metastable state and the state to be excited when relaxing from the metastable state to the state to be excited.
The lifetime may be about 80 μs. The lifetime may also be in the range of about 1 μs to about 10 ms.
Advantageously, the lifetime of the first intermediate state is long enough so that the first intermediate state can be populated using low optical powers, simultaneously, short enough so that after an exposure, the photoinitiator molecules outside the focal region quickly return to their ground state and subsequent focal regions may be exposed in rapid succession, as described further below in more detail. More generally, it is advantageous that the amount of photoinitiator molecules excited to the first intermediate state always stays small compared to the amount of photoinitiator molecules in the ground state, i.e., it is advantageous that there is no population inversion.
The term “single-photon excitation” refers to excitations which are caused by hitting an electron by one single photon and/or absorbing the photon by the electron. To excite an electron by one single photon, the photon needs to have a specific wavelength and/or frequency corresponding to the excitation energy of the electron, i.e., the energy difference between an initial state and the state to be excited.
In a specific embodiment, the polymerization-inducing state may be optically excitable by at least two sequential single-photon excitations of substantially the same predetermined wavelength.
Advantageously, in the aforesaid embodiment, the photoresist composition provides a comparable nonlinear transition rate scaling regarding the intensity I from the ground state to the polymerization-inducing state of I2 as in the case of two-photon absorption-based 3D-printing. As a result, the photoresist composition enables 3D-printing at high resolution and/or high printing velocity. By using the photoresist composition in 3D-printing, the spatial localization of optical excitations may be increased, whereby the resolution of 3D-printing may be improved.
The term “substantially the same” means that the wavelengths of the two subsequent photons causing the sequential two single-photon excitations may absolutely differ by about 0%, by utmost about 1%, by utmost about 5%, by utmost about 10%, or by utmost about 20%. However, in principle, it is also possible that the wavelengths of the two subsequent photons causing the sequential two single-photon excitations differ by more than about 20%.
The two sequential single-photon excitations may herein also be referred to as two-step absorption. Three sequential single-photon excitations may be referred to as three-step absorption. Four sequential single-photon excitations may be referred to as four-step photon absorption, and so on.
The aforementioned respective terms may be used interchangeably.
In a specific embodiment, the predetermined wavelength may be at least about 300 nm and/or utmost about 900 nm. The predetermined wavelength may also be at least about 400 nm and/or utmost about 800 nm. The predetermined wavelength may be about 405 nm, about 440 nm, about 488 nm, about 532 nm, or about 640 nm.
Additionally or alternatively, the single-photon excitation may have a predetermined frequency of at least about 0.3 PHz and/or utmost about 1 PHz. The single-photon excitation may also have a frequency of at least about 0.375 PHz and/or utmost about 0.75 PHz. The frequency may be about 0.74 PHz, about 0.68 PHz, about 0.61 PHz, about 0.56 PHz, or about 0.47 PHz.
Additionally or alternatively, the single-photon excitation may be of at least about 1.24 eV and/or utmost about 4.14 eV. The single-photon excitation may also be of at least about 1.55 eV and/or utmost about 3.11 eV. The single-photon excitation may be of about 3.07 eV, about 2.82 eV, about 2.53 eV, about 2.32 eV, or about 1.95 eV.
The wavelength may be the wavelength of the light emitted by a laser used in a Blu-ray player, a DVD player, a Blu-ray writing device, a DVD writing device, or a laser pointer.
The first intermediate state may be a real state.
In a specific embodiment, the first intermediate state may be a relaxed state with respect to an energetically higher state being optically excitable from the ground state by a single-photon excitation of the predetermined wavelength.
In this embodiment, the first intermediate state may be a state which is reachable by an intersystem crossing of the photoinitiator. The first intermediate state may be a state of different orientation of total spin or spin multiplicity than the energetically higher state. The first intermediate state may, for example, be a triplet state, whereas the energetically higher state may be a singlet state.
The photoinitiator may comprise multiple intermediate states as described above. All features described with respect to the first intermediate state apply to each of the multiple intermediate states.
The photoinitiator may have a second intermediate state energetically lower than the first intermediate state and lower than the polymerization-inducing state, wherein the second intermediate state may be substantially optically excitable from the ground state, and wherein the first intermediate state may be optically excitable from the second intermediate state based on a single-photon excitation of substantially the predetermined frequency.
The photoinitiator may have a third intermediate state energetically lower than the second intermediate state and lower than the polymerization-inducing state, wherein the third intermediate state may be substantially optically excitable from the ground state, and wherein the second intermediate state may be optically excitable from the third intermediate state based on a single-photon excitation of substantially the predetermined frequency, and so on.
The photoinitiator may have a linear, molar extinction coefficient in the ground state of about 0.1 to about 1000 M−1 cm−1 at the predetermined wavelength. The photoinitiator may have a linear, molar extinction coefficient in the ground state of about 50 M−1 cm−1 at the predetermined wavelength.
The photoinitiator may have a linear, molar extinction coefficient in at least one of the multiple intermediate states of about 10 to about 100 000 M−1 cm−1 at the predetermined wavelength. The photoinitiator may have a linear, molar extinction coefficient in at least one of the multiple intermediate states of about 5000 M−1 cm−1 at the predetermined wavelength.
The multiple intermediate states may have an energy of utmost about 70 kcal/mol. The multiple intermediate states may have an energy of utmost about 60 kcal/mol.
Advantageously, thereby, bond cleavage reactions may be prevented.
At least one of the multiple intermediate states may be inert.
In a specific embodiment, the photoinitiator may be selected from the group consisting of alpha-diketones such as benzil, beta-diketones, gamma-diketones, spiropyrans, merocyanines, carbazoles, thiophenes, polycyclic aromatic hydrocarbons, triketones, photoenoles, (di-)acylgermanes, bis(germyl)ketones, and thioxanthones.
In a specific embodiment, the polymerization inhibitor may be selected from the group consisting of 2,2,6,6-tetramethyl-4-piperidyl-1-oxyl (TEMPO) and derivatives thereof, bis (2,2,6,6-tetramethyl-4-piperidyl-1-oxyl) sebacate (BTPOS) and derivatives thereof, other hindered amine light stabilizers (HALS), 1,4-diazabicyclo[2.2.2]octane (DABCO), n-propyl gallate (NPG), p-phenylenediamine (PPD), cyclodextrines, phenothiazines, hydroxylamines, quinones, mequinol, 4-tert-butylcatechol (TBC), butylated hydroxytoluene (BHT), nitrobenzenes, phenol, p-nitrophenol, stilbenes, galvinoxyl, and azulene.
In a specific embodiment, the content of the photoinitiator may be 0.1% to 10% by weight based on the photoresist composition. Additionally or alternatively, the content of the polymerization inhibitor may be 0.05% to 10% by weight based on the photoresist composition.
The photoinitiator may have a plurality of absorption bands at at least about 300 nm and utmost about 900 nm. The photoinitiator may have a plurality of absorption bands at at least about 400 nm and utmost about 800 nm. The photoinitiator may have at least one absorption band at about 405 nm and/or about 440 nm and/or about 488 nm and/or about 532 nm and/or about 640 nm. The plurality of absorption bands may overlap.
Another aspect of the present invention relates to a system, in particular for 3D-printing, comprising:
Advantageously, the aforesaid system allows for cost-efficient and highly accurate 3D-printing, in particular on the scale of micrometers and/or nanometers, being extendable to larger scales. Furthermore, the system may be manufactured in a more compact fashion.
In a specific embodiment, the at least one continuous-wave light source may comprise a laser and/or a light emitting diode. The at least one continuous-wave light source may be a laser and/or a light emitting diode. The at least one continuous-wave light source may comprise or be a laser used in a Blu-ray player and/or a DVD player and/or a Blu-ray writing device and/or a DVD writing device and/or a laser pointer. The at least one continuous-wave light source may also comprise or be any suitable lamp, for example a mercury vapor lamp, a metal-halide lamp, or the like.
The at least one continuous-wave light source may comprise or be a non-pulsed laser. The at least one continuous-wave light source may include quasi continuous-wave light sources which are continuous-wave light sources that may be periodically interrupted.
In a specific embodiment, the light emitted by the at least one continuous-wave light source may have a wavelength of at least about 300 nm and/or utmost about 900 nm. The predetermined wavelength may also be at least about 400 nm and/or utmost about 800 nm. The predetermined wavelength may be about 405 nm, about 440 nm, about 488 nm, about 532 nm, or about 640 nm.
Additionally or alternatively, the light emitted by the at least one continuous-wave light source may have a frequency of at least about 0.3 PHz and/or utmost about 1 PHz. The light emitted by the at least one continuous-wave light source may also have a frequency of at least about 0.375 PHz and/or utmost about 0.75 PHz. The frequency may be about 0.74 PHz, about 0.68 PHz, about 0.61 PHz, about 0.56 PHz, or about 0.47 PHz.
The at least one continuous-wave light source may have an optical power of about 1 mW or larger. The at least one continuous-wave light source may also have an optical power in the range of about 1 mW to about 1 W, e.g., about 100 mW. The at least one continuous-wave light source may further have an optical power of about 1 W or larger.
The optical power of the at least one continuous-wave light source may be at least large enough to trigger the polymerization reaction inside the spatial location of the focus of the emitted light. For example, the optical power used for irradiating the photoresist composition in the spatial location of the focus of the emitted light may be about 1 mW or smaller, 100 μW or smaller, or 50 μW or smaller, without, however, being limited thereto.
The at least one continuous-wave light source may include 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10 continuous-wave light sources.
The system may further comprise a focusing unit for focusing the light emitted by the at least one continuous-wave light source on the photoresist composition. Specifically, the focusing unit focuses the light emitted by the at least one continuous-wave light source and sets a first spatial location of the focus of the emitted light, wherein the first spatial location of the focus corresponds to a volume element of the photoresist composition.
The focusing unit may comprise one or more of the following: a collimator, a plano-convex lens, a pinhole, a mirror, a polarizer, and an objective lens.
The system may also further comprise a positioning unit for controlling the position of the focusing unit with respect to the position of the photoresist composition. Specifically, the positioning unit spatially controls the focus of the emitted light such that the focus of the emitted light is movable from the first spatial location to a second spatial location of the focus, wherein the second spatial location of the focus corresponds to a second volume element of the photoresist composition, wherein the first volume element is substantially different from the second volume element. Positioning may be accomplished by moving the focusing unit and/or by moving the photoresist composition.
Herein, the focus of the light emitted by the at least one continuous-wave light source refers to a spatial region, also referred to herein as focal region, in which the light intensity and/or light power of the light emitted by the at least one continuous-wave light source is focused or concentrated. The focal region may be substantially two-dimensional. It may also be three-dimensional.
The first volume element and the second volume element may each correspond to the focal region. The first volume element and the second volume element may also over-lap.
The positioning unit may comprise a mirror arrangement and/or a stage. The stage is where the photoresist composition may be positioned. The mirror arrangement may comprise one or more fixed mirrors, and/or one or more motorized mirrors. The motorized mirrors may be piezoelectric, galvanometric or MEMS (micro-electro-mechanical systems) actuated mirrors. The stage may be motorized and may comprise stepper or servo motors, or piezoelectric, voice coil, or other magnetic actuators. The positioning unit may comprise fixed and/or motorized mirrors, and/or a fixed and/or motorized stage.
The system may be a confocal microscope.
Yet another aspect of the present invention relates to a method for producing a three-dimensional structure, wherein the method comprises the following steps:
Advantageously, by this method, a three-dimensional structure, in particular on the micro- and/or nanoscale, being extendable to larger scales, may be produced or manufactured with high resolution and in a cost-efficient and resource-saving manner.
The method may for example be performed as follows:
In a first step, the photoresist composition may be provided by placing or positioning it on a stage of a system according to the above-mentioned respective aspect or any one of the specific embodiments.
In a second step, at least one of the at least one continuous-wave light sources is switched on to irradiate at least a partial volume of the photoresist composition which initiates the polymerization of the polymerizable polymers comprised by the photoresist composition.
The at least partial volume may be a volume element in which the light intensity and/or light power of the light emitted by the at least one continuous-wave light source is focused or concentrated. The at least partial volume may be a volume element corresponding to the focal region of the light emitted by the at least one continuous-wave light source. The at least partial volume may also be the total volume of the photoresist composition.
In a specific embodiment, the method may further comprise removing an unpolymerized and/or incompletely polymerized remaining volume of the photoresist composition, wherein the polymerized at least partial volume of the photoresist composition may correspond to the three-dimensional structure.
The three-dimensional structure may be on the scale of micrometers and/or nanometers, being extendable to larger scales.
In case of negative-tone photoresists, the removing may include wet-chemical removing of the unpolymerized and/or incompletely polymerized remaining volume of the photoresist composition in a development solution. In case of positive-tone photoresists, the removing may include wet-chemical removing of the solubilized volume of the photoresist composition in a development solution.
In a specific embodiment, the step of causing polymerization may further include exciting the polymerization-inducing state of the photoinitiator of the photoresist composition by at least two sequential single-photon excitations of substantially the same predetermined wavelength.
Advantageously, in this embodiment, a three-dimensional structure may be produced with high resolution. The two-sequential single-photon excitations are spatially localized so that the resolution in producing the three-dimensional structure may be increased.
The above-described method may be performed by means of the above-described system according to the respective aspect or any one of the specific embodiments.
The step of causing polymerization may include irradiating a first volume element of the at least partial volume of the photoresist composition by focusing, by means of the focusing unit, the light emitted by the at least one continuous-wave light source on the first volume element.
The step of causing polymerization may further include moving, by means of the positioning unit, the spatial location of the focus of the light emitted by the at least one continuous-wave light source such that a second volume element spatially different from the first volume element is irradiated.
As already mentioned above, the first volume element and the second volume element may each correspond to the focal region of the light emitted by the at least one continuous-wave light source. The first volume element and the second volume element may also overlap.
Yet another aspect of the present invention is a use of a photoresist composition according to the above-described aspect or any one of the specific embodiments in 3D-printing.
Advantageously, by using the photoresist composition in 3D-printing, a three-dimensional structure, in particular on the microscale and/or nanoscale, being extendable to larger scales, may be produced or manufactured with high resolution, thereby allowing for small minimal feature sizes, in a cost-efficient and resource-saving manner.
In the following, the drawings are described. The drawings provide further explanations and show specific embodiments of the present invention. Individual features depicted in the drawings may be combined to yield further embodiments. In the drawings, it is shown:
In the following, the terms “photoresist composition” and “photoresist” may be used interchangeably.
In two-photon lithography illustrated in
The basic idea of two-step absorption shown in
We illustrate the more complex behavior of two-step absorption by the rate-equation calculations for the simplified energy-level model in
For benzil shown in
For Norrish Type I photoinitiators, radicals are generated by fragmentation of molecules into radicals by cleaving chemical bonds, with typical bond energies being above 3 eV (69 kcal/mol). The triplet ground state energy of benzil ET1=2.3 eV (54 kcal/mol), is well below that energy. For efficient bond scission from the triplet state, the triplet energy must be higher than the bond energy. Hence, benzil has been referred to as “reluctant” Norrish type I initiator—which has been our motivation for investigating benzil as a candidate for two-step absorption in the first place. However, benzil, as well as other alpha-diketones, readily abstracts hydrogen atoms from its triplet ground state. This leads to an undesired one-photon triggered polymerization reaction.
In addition to the two-step photoinitiator, the photoresist compositions used here and abbreviated as PR1 to PR4 in what follows, contain bis(2,2,6,6-tetramethyl-4-piperidyl-1-oxyl) sebacate (BTPOS) (see
PR1: Benzil (21.0 mg, 100 μmol) and BTPOS (25.6 mg, 50 μmol) are dissolved in 1 ml PETA. This corresponds to 1.7 wt % benzil and 2.1 wt % BTPOS. The mixture is stirred on a hot plate at a temperature of 45° C. for 4 hours until all compounds have dissolved.
PR2: Benzil (21.0 mg, 100 μmol) and BTPOS (25.6 mg, 50 μmol) are dissolved in a 1 ml mixture of IP-DIP NPI (Nanoscribe GmbH & Co. KG) and PETA (70 vol % and 30 vol %). This corresponds to 1.8 wt % benzil and 2.2 wt % BTPOS. The ratio of IP-DIP NPI and PETA is chosen to obtain a diffraction-limited focus. The mixture is stirred on a hot plate at a temperature of 45° C. for 4 hours until all compounds have dissolved.
PR3: Benzil (21.0 mg, 100 μmol) and BTPOS (25.6 mg, 50 μmol) are dissolved in 1 ml TMPTA. This corresponds to 1.8 wt % benzil and 2.2 wt % BTPOS. The mixture is stirred on a hot plate at a temperature of 45° C. for 4 hours until all compounds have dissolved.
PR4: Irgacure 369 (18.3 mg, 50 μmol) and BTPOS (12.8 mg, 25 μmol) are dissolved in 1 ml PETA. This corresponds to 1.5 wt % Irgacure 369 and 1.1 wt % BTPOS. The mixture is stirred on a hot plate at a temperature of 40° C. for 8 hours until all compounds have dissolved. The solubility of the Irgacure initiator was lower than that of benzil. The relative concentration of BTPOS was chosen to be constant. All photoresist compositions or photoresists were mixed and polymerized in yellow-light conditions.
Benzil (98%) was purchased from Aldrich. Bis(2,2,6,6-tetramethyl-4-piperidyl-1-oxyl) sebacate (BTPOS, 98%) was purchased from TCI chemicals. A ground state extinction spectrum is given in
To connect the behavior of our photoresist compositions with the modelling for two-step absorption in
For the experiments shown in
In fact, in control experiments on photoinitiators for which it is established that one gets one-photon absorption at the considered wavelength, we find changes in the slope as well. Corresponding experiments for Irgacure 369 are shown in
In general, one cannot draw conclusions from point-exposure experiments alone, regardless of the excitation mechanism. Consider the following example: A complex dense 3D structure requires many sequential point exposures. The tails of the laser focus excite photoinitiator molecules in a volume that is much larger than that of the printed structure. This generally means that ingredients of the photoresist, especially oxygen, are locally irreversibly consumed during the printing process. Diffusion of these molecules leads to a global reduction of their density. Thereby, a single point exposure influences even remotely separated other point exposures, giving rise to the proximity effect. Indeed, we are essentially unable to 3D-print structures using Irgacure 369, as shown in
In two dimensions, dense periodic line gratings (see
In
It is interesting to compare the scan speeds and laser powers used in two-step absorption with previous work on 3D laser printing using (quasi-)continuous-wave lasers exploiting one-photon absorption and chemical nonlinearities or two-photon absorption.
In conclusion, two-step absorption in benzil replacing two-photon absorption may be considered as the fundamental photoinitiation mechanism in 3D laser nanoprinting. The presented 2D and 3D benchmark nanostructures match or even surpass previous results obtained with two-photon absorption in terms of achievable spatial resolution. More importantly, our two-step absorption results were obtained with a continuous-wave semiconductor laser diode that is many orders of magnitude less expensive and smaller in volume than the femtosecond lasers commonly used today for obtaining efficient two-photon absorption. Moreover, for two-step absorption, we have not once observed an indication of micro-explosions. Unwanted photoresist micro-explosions typically occur every day when using two-photon absorption and femtosecond pulses in 3D laser nanoprinting.
The present inventors consider two-step absorption replacing two-photon absorption in 3D laser nanoprinting as a breakthrough. First, it paves the way to cutting down the cost of future 3D laser nanoprinters by several orders of magnitude—while maintaining and even surpassing the accessible spatial resolution, albeit currently at lower peak printing rates than best values obtained for two-photon absorption. Second, dramatic miniaturization of 3D laser nanoprinters is straightforward given that the diode laser used here has a volume of only some cubic millimeters. Combined, the drastic cost and size reduction will likely dramatically amplify the spread of 3D laser nanoprinters in the industrial field in the upcoming years. Third, by combining STED and two-step absorption, the achievable spatial resolution could be further improved, even beyond the diffraction barrier. Fourth, replacing one-photon absorption by two-step absorption could improve the spatial resolution in computed axial lithography. Furthermore, two-step absorption with two different laser colors is a prerequisite for light-sheet 3D laser printing. Both have the potential to boost 3D-printing speed.
In
In a more specific embodiment (not shown), the laser diode (L405P150, Thorlabs GmbH) is mounted on a temperature-controlled mount (LDM56/M, Thorlabs GmbH). A spectrum of the laser emission is displayed in
During 3D-printing, the sample is illuminated using an LED with a peak wavelength λ≈640 nm and observed in-situ in transmission-mode on a CCD camera.
The laser power was adjusted using a half-wave plate (WPHSM05-405, Thorlabs GmbH) and a polarizing beamsplitter cube (PBS251, Thorlabs GmbH), both located in front of the galvanometric mirror pair. In order to probe exposure powers even below the lowest possible power setting permitted by the half-wave plate and the beamsplitter, absorptive neutral density glass filters (FSQ-OD20 to FSQ-OD300, Coherent Inc.) were placed behind the polarizing beamsplitter cubes. The laser focus was checked routinely to ensure that the filter glass does not cause any wavefront aberrations.
All laser power values were measured at the objective lens' entrance pupil location using a semiconductor sensor (TP86, Coherent Inc.). For the point-exposure measurements, the pulses were monitored using an avalanche photodiode (APD410A/M, Thorlabs GmbH) placed at the idle output of the polarizing beamsplitter cube.
All samples except the woodpile photonic crystals were developed by immersing the sample for 3 minutes in a bath of acetone (UV/IR grade) and a subsequent wash in 2-propanol (UV/IR grade). The samples were then blow-dried in a gentle stream of nitrogen.
The woodpile photonic crystals were developed in acetone (UV/IR grade) with subsequent supercritical drying in CO2 using the EM CPD300 (Leica Microsystems GmbH).
Summing up, the quadratic optical nonlinearity arising from two-photon absorption provides the crucial spatial concentration of optical excitation in 3D laser nanoprinting, with widespread applications in the technical and life sciences. Femtosecond lasers allow for obtaining efficient two-photon absorption but are accompanied by a number of issues, including higher-order processes, cost, reliability, and size.
The present inventors have recognized that involving at least two sequential single-photon excitations or two-step photon absorption replacing two-photon absorption as the primary optical excitation process may overcome the above-mentioned issues and drawbacks of state-of-the-art 3D-printing. Two-step absorption may enable the same quadratic optical nonlinearity as two-photon absorption. Hence, the present invention allows for printing state-of-the-art 3D nanostructures and beyond with high resolution and opens the door to drastic miniaturization and cost cutting of 3D-printing.
Further, the present invention relates to the following items 1 to 15:
1. A photoresist composition, comprising:
2. The photoresist composition according to item 1, wherein the polymerization-inducing state is optically excitable by at least two sequential single-photon excitations of substantially the same predetermined wavelength.
3. The photoresist composition according to item 1 to 2, wherein the predetermined wavelength is at least about 300 nm and/or utmost about 900 nm, in particular at least about 400 nm and/or utmost about 800 nm.
4. The photoresist composition according to any one of items 1 to 3, wherein the first intermediate state is a relaxed state with respect to an energetically higher state being optically excitable from the ground state by a single-photon excitation.
5. The photoresist composition according to any one of items 1 to 4, wherein the photoinitiator is selected from the group consisting of alpha-diketones, beta-diketones, gamma-diketones, spiropyrans, merocyanines, carbazoles, thiophenes, polycyclic aromatic hydrocarbons, triketones, photoenoles, (di-)acylgermanes, bis(germyl)ketones, and thioxanthones.
6. The photoresist composition according to any one of items 1 to 5, wherein the polymerization inhibitor is selected from the group consisting of 2,2,6,6-tetramethyl-4-piperidyl-1-oxyl (TEMPO) and derivatives thereof, bis(2,2,6,6-tetramethyl-4-piperidyl-1-oxyl) sebacate (BTPOS) and derivatives thereof, other hindered amine light stabilizers (HALS), 1,4-diazabicyclo[2.2.2]octane (DABCO), n-propyl gallate (NPG), p-phenylenediamine (PPD), cyclodextrines, phenothiazines, hydroxylamines, quinones, mequinol, 4-tert-butylcatechol (TBC), butylated hydroxytoluene (BHT), nitrobenzenes, phenol, p-nitrophenol, stilbenes, galvinoxyl, and azulene.
7. The photoresist composition according to any one of items 1 to 6, wherein the content of the photoinitiator is 0.1% to 10% by weight based on the photoresist composition and/or the content of the polymerization inhibitor is 0.05% to 10% by weight based on the photoresist composition.
8. A system, in particular for 3D-printing, comprising:
9. The system according to item 8, wherein the at least one continuous-wave light source comprises a laser and/or a light emitting diode.
10. The system according to item 8 or 9, wherein the light emitted by the at least one continuous-wave light source has a wavelength of at least about 300 nm and/or utmost about 900 nm, in particular of at least about 400 nm and/or utmost about 800 nm.
11. The system according to any one of items 8 to 10, wherein the at least one continuous-wave light source has an optical power of about 1 mW or larger.
12. A method for producing a three-dimensional structure, wherein the method comprises the following steps:
13. The method according to item 12, wherein the method further comprises:
14. The method according to item 12 or 13, wherein the step of causing polymerization further includes:
15. Use of a photoresist composition according to any one of items 1 to 7 in 3D-printing.
| Number | Date | Country | Kind |
|---|---|---|---|
| 21205242.7 | Oct 2021 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/079954 | 10/26/2022 | WO |
