Development of a new photoresin

The key to 3D deep sub-diffraction OBL is the development of a unique material with two chemical activation channels. One is for photopolymerization and the other is for photoinhibition. For this aim, the material should be designed to satisfy the following requirements. First, it should include an initiator that is highly photosensitive to two-photon absorption generated by a writing beam, which allows for the near-threshold fabrication. Accordingly, it is possible to achieve a minimum degree of photopolymerization required for building solidified structures with a feature size smaller than the focal spot of the writing beam. Second, it should exhibit an effective inhibition of the 2PP process, which is achieved by a doughnut-shaped inhibition beam in a wavelength region different from the writing beam. The cross excitation between the writing and the inhibition beams, which means the excitation of the inhibitor by the writing beam or the excitation of the initiator by the inhibition beam, should be avoided. Third, the threshold intensity I th for the writing beam to generate 2PP should be as low as possible to avoid photodamage and uncontrolled thermal process. Fourth, it should exhibit the sufficient mechanical strength so that the structures fabricated at the near-threshold condition can survive during the wash-out developing process and withstand the unavoidable stress.

A critical development of the new photoresin was the appropriate selection of a photoinitiator that satisfies conditions 1, 2 and 3. We chose 2,5-bis(p-dimethylaminocinn amylidene)- cyclopentanone (BDCC) (Fig. 2a), which is an example of the D-π-A-π-D class of non-linear dyes. We examined π-conjugated ketone derivatives of this class11,12,13 capable of two-photon excitation. They have electron-donating diakylamino groups and carbonyl groups as the electron-withdrawing (acceptor) group that also allows hydrogen abstraction for subsequent initiation of free radical polymerization. Moreover, the length of the π-conjugated chain in between donor and acceptor parts allowed the tuning of the position of the absorption band into the visible region (Fig. 2b, red curve) and led to the enhancement of the two-photon cross-section (Fig. 2b, blue curve). The red-shifted absorption band of BDCC facilitated the inhibition process when tetraethylthiuram disulphide (TED, see Supplementary Fig. S1) was adopted as an ultraviolet inhibitor. It was evident that the absorption peak of BDCC was shifted to 511 nm from 360 nm of bis(4-(dimethylamino) phenyl)methanone (BDEP; Supplementary Fig. S2) or from 480 nm of 2,5-bis-[4-(dimethylamino)-benzylidene] -cyclopentanone (BDMA; Supplementary Fig. S3), which both had shorter lengths of π-conjugation. The corresponding 800 nm wavelength two-photon cross-section of BDCC was enhanced to 300 GM from 128 GM of BDEP or from 225 GM of BDMA. In the meantime, the laser intensity threshold I th for 2PP with BDCC was reduced to 75.75 mW cm−2 from 303.03 mW cm−2 (BDEP) or from 189.39 mW cm−2 (BDMA) at the scanning speed of 160 μm s−1.

Figure 2: Two-photon initiator and monomers. (a) The molecular structure of BDCC. The BDCC has the properties of low ultraviolet absorption and large two-photon absorption cross-section. (b) Single-photon absorption spectra of BDCC in chloroform and its two-photon cross-section obtained by the Z-scan method. (c) The monomers used in the photoresin are SR399, which has high functionality and can lead to fast curing, and SR444, which has low viscosity. Full size image

For the selection of monomers, a five-functional monomer, SR399 (Dipentaerythritol pentaacrylate, Fig. 2c), was used to improve the chemical reactivity for high photosensitivity and mechanical strength against the damage and the shrinkage (Supplementary Fig. S4 and Supplementary Discussion) during the developing process. Compared with other less-functional monomers, this five-functional monomer can provide a higher crosslinking density to reduce the gelation threshold and increase the mechanical strength for a given photopolymerization monomer conversion. Another trifunctional monomer, SR444 (pentaerythritol triacrylate, Fig. 2c), was introduced to keep the viscosity from decreasing the photoinhibition efficiency. These two multi-functional monomers support sufficient gelation with the intensity of the writing beam below the level of the photodamage and the unwanted thermal process. With a standard initiation system of camphorquinone (CQ) and ethyl 4-(dimethylamino)benzoate (EDAB) as co-initiator, the photosensitivity of the final photoresin (see ‘Material synthesis’ in Methods) was confirmed to be at 0.18 g J−1, implying at least two orders of magnitude higher than that of the formulation used in the single-photon fabrication9.

Feature size reduction with 3D deep sub-diffraction OBL

With our new two-photon absorption resin, which facilitates an immediate threshold operation (Fig. 3a), the 800-nm wavelength laser beam (see ‘Experimental setup’ in Methods and Supplementary Fig. S5) can produce not only a dot size of 95 nm (not shown here) but also the free-standing nanowires. Without the inhibition beam, the feature size, that is, the linewidth of the free-standing nanowire (Supplementary Fig. S6) could be reduced from 150 nm (the black point in Fig. 3b) to 42 nm (point A in Fig. 3b), when the writing beam-dosed energy was decreased from 8E min to E min (There exists a minimum energy dose E min for maintaining the fabricated polymer feature against destructive forces. Here, the E min is the experimentally observed minimum energy dose required to survive the fabricated free-standing nanowires). The smallest nanowire linewidth of 42 nm (point A in Fig. 3b) was achieved in our new resin at the writing energy of E min . A further decrease of the writing beam energy dose decreased the degree of photopolymerization at the focal centre, leading to the disintegration of the fabricated structures after the developing process. This result indicates a critical limitation in the OBL due to the existence of the material threshold in the focal centre, resulting from the minimum monomer conversion required to compete with deconstructive forces, such as the change of the surface tensions.

Figure 3: Feature size measurement. (a) Schematic demonstration of the feature size reduction in single-beam OBL by decreasing the writing laser beam energy dose to the polymerization degree threshold of the photoresin. The black curve corresponds to the black point in b. The red curve corresponds to the point A in b. (b) Feature size versus the writing laser beam energy dose for the new resin without the inhibition beam in the experiment. (c) Schematic illustration of feature size reduction in two-beam OBL by increasing the intensity of the inhibition beam. The effective focal spot profile becomes narrower by increasing the intensity of the inhibition laser beam with the fixed irradiation intensity of the writing beam. The blue arrow suggests the direction of increasing the intensity of the inhibition laser beam. (d) Feature size of free-standing lines versus the intensity of the inhibition beam, under the exposure of the writing beam with different energy doses. The dots with error bars represent the experimental results. Curves guiding the dots are the fitting with the formula: . The insert pictures show the scanning electron microscopy images of points A, B, C, D and E with a scale bar of 100 nm. All the error bars reflect s.d. Full size image

While a doughnut-shaped inhibition beam is introduced into two-beam OBL (Supplementary Fig. S5), the photoinhibition process is activated to confine the photopolymerization to the centre of the focal spot through the photo-excitation of the inhibitors (TED). As the inhibition process is achieved immediately near the threshold of the new material while the intensity of the writing beam at the focal centre remains the same as if there was no inhibition beam, the effective intensity profile (that is, the effecive focal spot size), equavalent to the area with the photopolymerization degree above the threshold becomes smaller (Fig. 3c). Thus, the photopolymerized feature size can be further reduced (Fig. 3c). As the photopolymerization is inhibited only in the ring of the doughnut-shaped inhibition beam in this method, the degree of the photopolymerization monomer conversion at the focus centre does not change and remains above the threshold, allowing the photopolymerized structure to survive after the developing process. With the inhibition beam of intensity levels of 0.69, 1.15, 1.62, 2.31 and 2.42 μW cm−2, the feature size, that is, the linewidth of the free-standing nanowires is experimentally reduced from 42 nm (point A in Fig. 3d) to 34 nm (point B in Fig. 3d), 18 nm (point C in Fig. 3d), 11 nm (point D in Fig. 3d) and 9 nm (point E in Fig. 3d), respectively, which agrees with the prediction by the theoretical numerical simulation (Supplementary Fig. S7). The experimental and numerical simulation data can be fitted with a formula (Supplementary Discussion9,14,15,16,17): line width (nm) = with α 1 =34.48 (nm), β 1 = 0.44 ((μW cm−2)−3) and I S defined as the inhibition beam saturation intensity. The α 1 value varies as the inhibition beam applied at different energy dose of the writing beam (as shown in Fig. 3d). The fabrication of lines with width below 50 nm using higher energy dose of the writing beam and higher inhibition beam intensity implies a large flexibility of the exposure latitude of the two beams, which greatly differs from single-beam OBL requiring critical exposure dose for obtaining small feature structures. A further analysis shows that the size dependence determined by with two-beam OBL holds in the axial direction (Supplementary Fig. S8 and Supplementary Disscussion).

Resolution improvement with 3D deep sub-diffraction OBL

Fabrication resolution is a different concept from feature size studied in Fig. 3 and can be determined by two-line resolution, which is the minimum centre-to-centre distance between the two fabricated lines. Though the condition for reducing the feature size is the necessary but not the sufficient condition for improving resolution, the realization of 9 nm feature size in two-beam OBL provides the prerequisite for the fabrication with the two-line resolution beyond the diffraction limit of the writing beam, as shown schematically in Fig. 4a. Without the inhibition beam, the two-line resolution achieved in the experiment is 246 nm (point A in Fig. 4b), which is approximately the diffraction limit of the writing beam at the wavelength of 800 nm.

Figure 4: Two-line resolution measurement. (a) Schematic demonstration of the two-line resolution improvement. Δ on and Δ off represent the two-line resolution with and without the inhibition laser beam, respectively. The dashed line indicates the threshold of the effective intensity. (b) Two-line resolution versus the intensity of the inhibition laser beam. The inserted scanning electron microscopy (SEM) image shows two adjacent lines fabricated with the inhibition laser beam switched off and on, respectively. The insert is the plot of the cross-sectional profile of the inserted SEM image. The error bars reflect s.d. (c) SEM images of points B, C, D, E, F and G shown in b. (d) The cross-sectional profile of image G in c. Scale bars, 100 nm. Full size image

Breaking this resolution limit requires the photo-excitation of a sufficient quantity of photoinhibitors with the inhibition beam (Supplementary Fig. S9 and Supplementary Disscussion). The over-loading of photoinhibitors can compensate the loss of its concentration caused by the repeated scanning of the doughnut-shaped inhibition beam during the fabrication. As the intensity of the inhibition beam increases to 0.57, 1.15, 1.44, 2.02 and 2.31 μW cm−2, accompanied with the decreasing linewidth, the resolution is improved from 246 nm to 205 nm (point B in Fig. 4b), 157 nm (point C in Fig. 4b), 117 nm (point D in Fig. 4b), 87 nm (point E in Fig. 4b) and 57 nm (point F in Fig. 4b), respectively. The best resolution achieved is 52 nm (point G in Fig. 4b) with an inhibition beam intensity of 2.42 μW cm−2, which is 1/7 of the inhibition beam wavelength and 1/5 of the resolution as achieved without the inhibition beam, as shown in Fig. 4d.

Fitting the experimental data (the red curve, which guides the blue experimental data in Fig. 4b) reveals the resolution dependence on the inhibition intensity: resolution in-plane (nm)= with α 2 =252.07 (nm), β 2 =2.04 ((μW cm−2)−1). This dependence clearly shows that the resolution is far beyond the diffraction limit of the writing beam. In fact, the formula indicates the possibility of the fabrication resolution in the nanoscale. Compared with the dependence of the linewidth on the inhibition intensity, , revealed in Fig. 3, the decrease rate of the two-line resolution is slower upon increasing the intensity of the inhibition beam, as is reflected by the different power indices of the inhibition beam intensity in the two formulas. This difference originates from the re-exposure of the photoresin for the fabrication of two adjacent lines, which requires higher intensity of the inhibition beam to suppress the polymerization in the space between these two lines than the single-line fabrication case. Further, fabricating more adjacent lines (in Fig. 5) can degrade the resolution as the exposure dose of the inhibition beam and the writing beam in a given region grows due to the repeated scans, which needs to restrain the polymerization between each line and compensate the consumption of photoinhibitors. With an inhibition beam intensity of 2.42 μW cm−2, the resolution of 90 nm was realized for the fabrication of ten parallel lines (Fig. 5c).