STED-inspired Laser Lithography Based on Photoswitchable

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STED-inspired Laser Lithography Based on Photoswitchable Spirothiopyran Moieties Patrick Müller, Rouven Müller, Larissa Hammer, Christopher Barner-Kowollik, Martin Wegener, and Eva Blasco Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b04696 • Publication Date (Web): 15 Jan 2019 Downloaded from http://pubs.acs.org on January 17, 2019

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Chemistry of Materials

STED-inspired Laser Lithography Based on Photoswitchable Spirothiopyran Moieties Patrick Müller,†,$,# Rouven Müller,◊,# Larissa Hammer,◊ Christopher Barner-Kowollik,◊,¥,* Martin Wegener,†,$,* Eva Blasco◊,* † $

Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT), 76344 Eggenstein-Leopoldshafen, Germany

Institute of Applied Physics, Karlsruhe Institute of Technology (KIT), 76128 Karlsruhe, Germany

◊

Macromolecular Architectures, Institute of Technical Chemistry and Polymer Chemistry, Karlsruhe Institute of Technology (KIT), 76128 Karlsruhe, Germany ¥

School of Chemistry, Physics, and Mechanical Engineering, Queensland University of Technology (QUT), Brisbane, QLD 4000, Australia ABSTRACT: We introduce a photoresist based on methacrylate copolymers bearing photochromic spirothiopyran moieties as side groups which can crosslink via supramolecular interaction between the chromophores. Upon two-photon excitation, the resist is capable of generating freestanding 3D structures and offers an inhibition channel, which allows for STEDinspired laser lithography. Reversible inhibition, linewidth narrowing, and resolution enhancement are demonstrated.

Introduction Light cannot be focused to an arbitrarily small spot. Instead, being a consequence of the wave nature of light, diffraction fundamentally limits the achievable resolution in both optical microscopy and optical fabrication techniques.1 Only in the last decades, the introduction of superresolution schemes in fluorescence microscopy, namely stimulated-emission depletion (STED) microscopy2 and localization techniques like PALM3 and STORM,4 allowed to break this barrier by using concepts of modern physics. The idea of STED microscopy relies on a molecular photoswitch, i.e., a molecule that can be switched between two states: i) from a non-fluorescent state to a fluorescent state by one wavelength of light, referred to as excitation, and ii) vice versa (from fluorescent to non-fluorescent) by a second wavelength, called depletion. The depletion wavelength is employed in a spatial focus shape that features a special point (or line or plane) of zero intensity at the position of the excitation intensity maximum. Thus, by increasing the depletion intensity, more and more molecules are forced into the non-fluorescent state, increasingly suppressing fluorescence in the vicinity of that special point. It is theoretically possible to confine the remaining number of molecules in the fluorescent state to an arbitrarily small volume the size of which is no longer limited by diffraction but ultimately only by the size of the employed molecule. This principle only relies on the effective behavior, but not on the microscopic mechanism of the photoswitch. In fact, for STED microscopy, different mechanisms have been reported, e.g., stimulated-emission depletion, photoinduced intersystem crossing into dark triplet

states, or switching between bistable molecular states (see review5). In order to achieve the necessary depletion beam shapes, dielectric or birefringent phase masks6 or interference patterns7 have been employed. More recently, the principle of STED microscopy has been adopted for a 3D laser lithography technique called direct laser writing (DLW).8 In conventional DLW, a tightly focused laser beam is scanned through a typically liquid negative-tone photoresist that is crosslinked by multi-photon absorption in a volume around the beam waist. By serially exposing the photoresist, almost arbitrary 3D structures can be fabricated, realizing 3D printing on the micro- and nanoscale. DLW has recently enabled many developments and applications in different fields including nanophotonics,9 cell biology,10 metamaterials,11 and microfluidics.12 Although this technique allows for submicron resolution, for some of the applications this is not sufficient and super-resolution beyond the diffraction limit is needed. A main challenge for introducing a STEDinspired super-resolution scheme is to identify materials working as the lithographic equivalent of a molecular photoswitch in STED microscopy. In this case, instead of fluorescence, it is the formation of covalent or non-covalent crosslinks that can be triggered and inhibited by different wavelengths of light.13 In the literature, several molecular systems based on different approaches have been reported that effectively realize such a mechanism.8 Most approaches focus on materials based on free-radical polymerization of acrylates. In conventional DLW, photoinitiator molecules are excited by multi-photon absorption, transit into a triplet state, and eventually decay into radicals, initiating

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Figure 1. a) The employed spirothiopyran-bearing copolymer in the closed form (SP) can be transferred into its open merocyanine form (MC) by two-photon absorption at 820 nm. The reverse photoisomerization (MC to SP) takes place upon irradiation at 640 nm. The MC form can create supramolecular links with other SP or MC moieties (the latter is depicted). b) Absorption spectra of SP (in dark) and MC (photostationary state) forms in acetophenone. c) A combination of a Gaussian excitation focus and depletion focus featuring a valley of zero intensity yields reduced sub-diffraction feature sizes.

polymerization and thereby covalently crosslinking of the photoresist. For some photoinitiators, e.g., diethylamino3-thenoylcoumarin, it is reported that while residing in the excited singlet state, the molecules can be brought back down to the ground state by stimulated emission using focused light at 532 nm, thus evading the pathway to radical formation.14 Similarly, using photoinitiators such as malachite green carbinol base15 or isopropyl thioxanthone,13 it is possible to avoid dissociation by excited-state absorption into higher triplet states, from where a non-radiative decay regenerates the ground state. Instead of deactivating photoinitiator molecules, a different approach aims on affecting the polymerization propagation.16 Here, quencher molecules that can be photoactivated by the depletion light are added to the resist, thereby inhibiting polymerization. In all these approaches, the depletion mechanism needs to compete with the efficient and fast dissociation process of the photoinitiator molecules, which usually takes place on the nanosecond timescale.17

an activated double bond, forming a covalent crosslink between the two moieties.21,22 By phototriggered isomerization of the (E)-enol into the corresponding (Z)-enol, the population of the (E)-enol can effectively be reduced, thereby inhibiting the cycloaddition. Along these lines, photochromic molecules offer interesting properties: They possess two (ideally) bistable forms which can transit into each other by exposing them to different wavelengths, as the two forms feature very different absorption spectra.23–25 However, the huge difference in absorption is often not accompanied by crosslinking, which is a crucial requirement for lithographic applications. A notable exception is the photochromic spirothiopyran (STP) molecule.26,27 In the dark, the thermodynamic more stable spirothiopyran form (SP) exists as the predominant species. It can be switched from the SP to a merocyanine (MC) form by exposure to UV light. The reverse reaction is feasible via either thermal relaxation or visible light irradiation (Figure 1a). The MC form features a thiolate anion which can undergo a thiol-Michael reaction with a second molecule carrying an activated ene, e.g., a maleimide, to form an irreversible covalent linkage.28–30 Vijayamohanan and et al. have recently proposed a STED-inspired super-resolution scheme for interference lithography employing a twocomponent photoresist based on spirothiopyran (STP) and maleimide functionalized polymers and proved writing inhibition in macroscopic experiments.31 Although the authors calculated the potential resolution increase in microscopic interference lithography, no experimental evidence has been shown.

Instead of free-radical polymerization, direct photocontrol of the individual crosslink formation has emerged as an interesting avenue. A system based on the photoinduced generation of o-quinodimethanes from o-methyl benzaldehydes (photoenolization)18 has recently been introduced by us for sub-diffraction surface functionalization.19 Two-photon-excited benzaldehydes form two different configuration isomers upon photoenolization, namely (E)- and (Z)-enol.20 The (Z)-enol features a short lifetime and quickly reverts to the ground state, while the longerlived (E)-enol can react in a Diels-Alder cycloaddition with

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Chemistry of Materials In this work, we introduce a simplified one-component photoresist based on supramolecular interactions of STP groups for STED-inspired DLW, employing two-photon excitation. In contrast to the study of Vijayamohanan et al., we experimentally demonstrate linewidth and resolution enhancement on the microand nanoscale.

excitation laser according to the procedure depicted in Figure 2a. The liquid photoresist

Results and Discussion In order to exploit the STP chromophore in STEDinspired DLW, we first synthesized methacrylate-based copolymers with STP functionalities as side groups (Figure 1a, for experimental details on the synthesis, please refer to Supporting Information). The polymer with the best writing performance (see discussion below) has a functionalization degree of 14 mol% STP and a Mn of 6100 g mol-1. Figure 1b shows the measured absorption spectra of the SP and MC form in the dark and in the photostationary state in acetophenone, which serve for selecting the ideal wavelengths for writing (excitation) and depletion. As excitation source, we used a mode-locked Ti:Sapphire laser oscillator, emitting femtosecond pulses with a center wavelength of 820 nm. This wavelength is well separated from the broad absorption spectrum of the open MC form between 600 nm and 800 nm, but is still capable of efficient two-photon excitation of the closed SP form. As depletion source, a continuous wave (cw) laser diode at 640 nm wavelength was employed (Figure 1c), which is suitable for exciting the broad MC band to photochemically trigger the MC to SP ring closure. The experimental setup used for DLW was described in previous publications17 and is here modified to include a 640 nm laser diode. The employed microscope objective lens has a numerical aperture of 1.4. First, 3D microstructures were prepared using solely the

Figure 2. a) Sample processing involves i) applying the liquid photoresist onto a glass substrate, ii) performing laser lithography, and iii) washing away the unexposed material in a development step. b) Scanning electron micrographs of freestanding 3D structures fabricated using the one-component photoresist based on spirothiopyran-bearing copolymer.

was applied onto a glass substrate before the desired structures were exposed serially by using a 3D piezo stage to scan the sample position relative to the fixed laser beam focus. After exposure, the unused photoresist was washed away by development in toluene followed by dipping in isopropyl alcohol and blowing with nitrogen.



Figure 3. False-color plots of laser focus point-spread functions measured by gold bead scattering experiments. Both laser beams can be employed in a Gaussian shape, where the depletion laser at 640 nm is intentionally slightly widened to match the beam width of the excitation laser at 820 nm. Additionally, a phase mask that imposes a phase shift of 𝜋 to one half-space of the depletion beam yields a focus with a line of zero intensity across the vertical direction in the center. Intensity profiles along the dashed lines are shown below.

MC form, which has a remarkably increased dipole moment compared to the SP structure.32 For the oxygen-containing analogue of the spirothiopyran, spiropyran, the corresponding merocyanine structures are well known to form strong intermolecular associates. Krongauz et al. described the formation of SP-MC dimers and charge-transfer complexes of small spiropyran molecules resulting in the formation of aggregates with a high degree of association (>106), leading to macroscopic structures in the micrometer range.33 In later work, aggregates of type SP-MC, MCn, and (SPnMCm) were described.34–37 For spiropyran containing methacrylate copolymers, a mechanism termed “zipper crystallization” was found to result in polymer crystals which were stable even at 150°C.38–41 More recently, the strong ability of spiropyrans for self-organization has been used for UV-induced aggregation of colloidal particles42 and in combination with multi-photon DLW for crosslinking of functionalized nanoparticles.43 In order to further evaluate

The photoresist allowed fabrication of well-defined 3D microstructures using only the excitation beam (Figure 2b). The structures show shrinkage because unused material initially also resided inside of the structures and was washed out in the development step. We attribute the formation of stable 3D structures in our resist to light-induced supramolecular linkages. It is well known that upon UV irradiation of the STP molecules, ring-opening and isomerization leads to the zwitterionic

Figure 4. Depletion experiments using overlaid Gaussian excitation and depletion foci. a) Varying both laser powers while writing lines shows two regimes: At low to intermediate depletion powers, the writing threshold power increases due to the depletion effect. At higher depletion powers, parasitic writing by the depletion laser lowers the threshold. b) The reversibility of the depletion effect is demonstrated by writing a loop pattern. Previously written lines are not altered by the depletion light (dashed area 1). It is also possible to write again in an area that has been irradiated by the depletion laser before (dashed area 2).

the validity of this interpretation, we synthesized an analogous copolymer bearing spiropyran groups (see Supporting Information page S8). Here, the MC form is not reactive and thus covalent cross-linking is not expected.

the weight of a single polymer chain. As a certain weight is necessary to cross the gelation point, i.e., to form an insoluble network, employing shorter copolymer chains leads to a higher degree of crosslinking in the final polymer network, yielding a more rigid polymer at the cost of a higher exposure dose.45 Second, by increasing the functionalization degree, the degree of crosslinking simultaneously increases due to a higher concentration of photoactive groups in a given volume element. However, there is a trade-off between functionalization degree and solubility as well as steric hindrance of neighboring STP groups.46 In preliminary writing experiments, we tested different copolymer compositions and solvents including toluene, acetophenone and dimethylformamide. The optimal photoresist consists of a copolymer with a functionalization degree of 14 mol% STP and a Mn of 6100 g mol-1, dissolved in acetophenone at 40 wt%.

In DLW, this model resist showed very similar lithographic performance compared to the STP-based resist (Figure S11). Thus, the combination of the above-described strong intra- and intermolecular aggregation behavior of STP-containing methacrylate copolymers results in formation of physically linked networks (Figure 1a). There are several degrees of freedom in the copolymer design, such as chain length or functionalization degree, i.e., the molar percentage of STP groups in the copolymer.44 Thus, the writing performance of different copolymer compositions was investigated (see Table S1). We observed two main trends: first, with each crosslink formed, the total weight of the polymer network approximately increases by


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Chemistry of Materials By imposing super-pulsing schemes onto the excitation laser and determining the writing threshold power,47 i.e., the excitation power needed to cross the gelation point, the effective order of nonlinearity was found to be 1.84, in agreement with the expected two-photon behavior (Figure S13).

Nevertheless, a considerable depletion effect is evident. In contrast, performing the same experiment using a copolymer containing the oxygen analogue spiropyran did not lead to a reversible depletion effect (see Figure S12). In a second test, a loop pattern was written with the excitation laser where the depletion laser was only switched on in the central part (Figure 4b). The result proves two necessary prerequisites for a useful depletion effect: i) previously written lines were not damaged by the depletion laser light (see dashed area 1 in Figure 4b) and ii) it was possible to write through a region that has already been irradiated with the depletion light (see dashed area 2 in Figure 4b). This proves that the excitation/depletion switching is reversible, i.e., the depletion light leads to reforming STP. These findings also support the above described mechanism of strong intra- and intermolecular aggregation, as writing is only feasible when the MC form is enriched. In addition, the dipolar interactions in the written structures are strong enough to withstand the depletion beam as well as the washing steps in toluene and isopropyl alcohol. By writing structures while time-gating both lasers sequentially, we found that the depletion laser did not have an effect anymore when the depletion light arrived > 4 µs after the excitation light (see Figure

As a next step, we performed depletion experiments. While the excitation laser focus always had a diffractionlimited Gaussian shape, different focus shapes were used for conducting depletion experiments. The foci were measured by scanning gold beads and recording the back-scattered light and are shown in Figure 3. For global depletion, a broadened Gaussian focus was used. For high-resolution experiments, a phase mask was inserted into the depletion beam, which imposed a phase shift of π onto one half-space of the beam, resulting in a focus that features a valley of zero intensity in its center. Using the global depletion arrangement, several line patterns were written, varying both the excitation and depletion laser power (Figure 4a). With increasing depletion power, the threshold power, is considerably increased. Above depletion laser powers of approximately 0.3 mW, the threshold power decreases again, which we attribute to parasitic one-photon absorption of the depletion laser.

Figure 5. a) Scanning electron micrograph of a typical set of lines where the depletion laser is switched on for the lower part of the line. Linewidths are reduced with increasing depletion laser power until the lines become unstable and eventually disconnected. b) The averaged profile extracted from the micrograph in a) (see dashed yellow line) shows a full width at half maximum (FWHM) of 31.2 nm. c) The FWHM of lines is proportional to the inverse square root of the depletion power expected in theory.

S14). However, this time scale for the network formation should be considered an upper limit due to electronic limitations.

similar results can be obtained for different excitation laser powers. With increasing depletion laser power, the line thickness decreases until a point is reached, where the lines are not continuous anymore. The minimum full linewidth at half maximum (FWHM) of an averaged line profile measured in the SEM is typically on the order of 30 nm to 35 nm. In the depicted case, the FWHM linewidth is 31.2 nm (Figure 5b).

To investigate the resolution improvement, the depletion beam was modulated by a phase mask, as mentioned above. A set of lines was written with a fixed excitation laser power, where the depletion laser is switched on during writing. From line to line, the depletion laser power was increased. Figure 5a shows a typical set of such lines, where



The FWHM approximately scales with the inverse square root of the depletion power, as shown in Figure 5c. This scaling behavior is expected for different STED-like processes.48 In this instance, the FWHM achievable without depletion is 104 nm, at a higher excitation power of 0.69 mW (50% above the writing threshold). The thinnest lines that could be fabricated with only the excitation beam showed a FWHM of around 55 nm, written with an excitation power of 0.46 mW, close to the threshold. However, at such low excitation powers, the depletion light directly yielded disconnected lines without leaving a processing window in which the line thinning effect could be observed. Overall, the achievable feature size was reduced by nearly a factor of two by employing depletion. Limiting factors are on one hand, the decreasing stability of thin lines, which lead to collapsing and breakup, and on the other hand, an inherent graininess of the crosslinked resist (Figure S15), with apparent grain sizes on the order of 30-40 nm. The origin of these grains remains unclear but could be connected to the formation of sub-micron crystallites due to highly ordered stacking of the MC moieties. In fact, similar grain sizes have been reported for zipper polymerization of spiropyran methacrylate copolymers.34

1.4 and is given by the two-photon-modified Abbe criterion, 𝜆/(2 ⋅ √2 ⋅ NA) = 207 nm. In fact, our experiments showed that gratings with periods a above this limit, in the depicted case a = 400 nm, could be easily fabricated in good quality. Below the diffraction limit, e.g., at a = 175 nm, using only the excitation beam, no clearly separated lines were feasible. In contrast, if the depletion laser modulated by the half-space phase mask was switched on, the lines could be resolved again. However, it is apparent that the grating quality tremendously suffered as numerous microbridges were formed between the lines. We attribute this to swelling effects during the development procedure, as is often observed for negative-tone photoresists across all kinds of microlithography processes.49–51 During development, the solvent swells both the unused polymer as well as the crosslinked network. The latter thereby strongly increases in volume, which in the case of gratings leads to neighboring lines touching each other and forming noncovalent links. When drying the sample, parts of these connections remain in the form of microbridges, while in other parts, the swelling leads to detachment from the substrate and further deterioration of the lines. This effect can be counteracted to some extent by using copolymer chains with a smaller molecular weight, thus increasing the degree of crosslinking. However, within the resist comparison study described above, no resist composition could be identified which completely averts this problem. While direct observation of swelling proved to be challenging because of the low contrast between cross-linked structures and photoresist, a

Having achieved a considerable feature size reduction, a second benchmark of interest is the resolution, i.e., the minimum distance between two features. To investigate this, line gratings were written with varying excitation powers and varying grating period (Figure 6). The theoretical limit due to diffraction depends on the excitation wavelength 𝜆 = 820 nm and the numerical aperture NA =

Figure 6. Scanning electron micrographs of different (successfully and unsuccessfully) fabricated gratings. Gratings written with lattice constants above the diffraction limit of 207 nm (for the employed setup) feature clearly separated lines. At lattice constants below the diffraction limit, lines can only be resolved if the depletion beam is switched on. The structure quality suffers from swelling-induced microbridging.

study of swelling-induced damage involving already developed structures supports our conclusion (Figure S16). By comparison, photoresists based on free-radical polymerization of acrylate monomers suffer less in this regard and allow for higher resolutions.8 Although overcoming these limitations is subject to further investigation, clear indications of sub-diffraction resolution were found.

We have introduced a one-component photoresist that consists of a copolymer of methyl methacrylate and STPfunctionalized methacrylate with acetophenone as suitable solvent. Upon two-photon absorption at 820 nm, the STP moieties transit into their open MC forms, which allow for interaction between SP and MC moieties from the same and neighboring polymer chains, leading to efficient physical entanglement of the copolymer chains. Utilizing this photoswitchable system, it was possible to fabricate freestanding 3D structures by DLW. We have demonstrated a

Conclusions 6

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Chemistry of Materials reversible depletion effect that can be used in STEDinspired DLW, employing a 640 nm cw depletion laser. By using a half-space phase mask for the depletion beam, the linewidth could be reduced by almost a factor of two, down to 31 nm, compared to diffraction-limited DLW in the same resist. Line gratings have also demonstrated the capability of the mechanism to support sub-diffraction resolution but are yet limited by swelling-induced deteriorations. We are confident that these limitations can be overcome in the future, e.g., by tailoring the backbone or the composition of the copolymers, especially with a view to the high level of maturity and success that copolymer-based photoresists have achieved in industrial 2D microlithography.52 The introduction of photochromic chromophores like STP paves the way for further resolution enhancements in STEDinspired laser lithography.

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Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Materials, characterization methods, experimental procedures, full spectroscopic data for all new compounds, copies of 1H and 13C{1H} NMR, HR-ESI-MS, SEC, DLW setup description, scanning electron microscopy, and light microscopy images of supplementary lithography experiments.

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Corresponding Author *[email protected] *[email protected] *[email protected] *[email protected]

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authors contributed equally.

Notes

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The authors declare no competing financial interest. (15)

ACKNOWLEDGMENTS M.W. and C.B.-K. acknowledge support from the Karlsruhe Institute of Technology (KIT) and the Helmholtz-Gemeinschaft in the context of the Helmholtz program Science and Technology of Nanosystems (STN). C.B.-K. acknowledges funding from the Australian Research Council (ARC) in the form of a Laureate Fellowship enabling his photochemical research program as well as key support from the Queensland University of Technology. C.B.-K. and M.W. acknowledge support for this project from the ARC in the form of a Discovery Grant focused on STED lithography (DPDP180100316). P. M. acknowledges support by the Karlsruhe School of Optics & Photonics (KSOP). Lukas Bangert is warmly thanked for assisting in synthesis.

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Chemistry of Materials

ToC Figure


STED-inspired Laser Lithography Based on Photoswitchable

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