Nanofabrication of structures with a feature size of sub-50 nm with ultrashort-laser based two-photon polymerization (2PP) technique is presented. The spatial resolution of the 2PP structures depends on the characteristics of the polymer material and the laser system used for fabrication. Here we compare the successful creation of sub-100 nm structures with two different few-cycle laser systems and chemically modified zirconium-based sol-gel composite material using cross-linker for resolution enhancement.
©2012 Optical Society of America
The two‐photon polymerization (2PP) technique is a well-established method for fabrication of high-quality 3D‐structures with nano-scale resolution. The most attractive advantage of this method is the potential to realize three-dimensional complex objects without geometrical limitations. So far, a large variety of photonic devices and nano-machines  have already been produced with feature sizes beyond the diffraction limit [2–6].
Usually, a tightly focused laser beam, with pulse durations between several tens to hundreds of femtoseconds or a few picoseconds, is used to fabricate the micro‐ and nanostructures directly inside the material by taking advantage of two-photon absorption . Inside the photosensitive material the localized polymerization occurs just in a very small area as a result of its cubic dependence , where the accumulated energy achieves the polymerization threshold at the focal spot of the incident laser beam. Due to two‐photon absorption and the threshold behavior of the used material , it is possible to reach higher resolution compared to the commonly used laser direct writing process [10,11]. With a precise control of the parameters like pulse number and pulse energy a spatial resolution markedly below the diffraction limit of the used laser wavelength could be reached. Even with laser radiation around 800 nm (diffraction limit around 400 nm) structure sizes below 100 nm are achievable. Publications of different groups show, that the 2PP technique allows for reaching a structuring resolution of sub-100 nm in various materials. Up to 65 nm thin lines were reported in triacrylate monomers with 520 nm and 730 nm excitation , in SU-8 photoresist 30 nm structures  and in SCR500 even sub-25 nm lines are reached  with pulse durations around 100 fs and typical Ti:sapphire central wavelengths around 800 nm.
Using very short laser pulses with the duration of just a few optical cycles (sub-10 fs in the VIS an NIR spectral range) and high repetition rates results in various benefits for 2PP. Based on the very high peak power of such short pulses the 2PP-threshold can be reached at a reduced average power resulting in an excellent adjustability allowing for very small voxel creation near the threshold. In combination with a high pulse repetition rate (> 0.2 MHz) the quality of photo polymerization rises with shorter pulses .
Here, we demonstrate 2PP with a new photosensitive, non-toxic inorganic–organic hybrid material, which is synthesized by adding a cross-linker to the zirconium based hybrid material (Zr-hybrid material). Caused by the cross-linker an enhanced stability of the polymerized material and an increase of the survival odds of the fragile structures during the post-treatment process can be expected . With the above mentioned benefits of the available few-cycle laser systems (a home-built non-collinear optical parametrical oscillator (NOPA) and a commercially available Ti:sapphire oscillator) in combination with the new sol-gel material we were able to create line structures with a minimum width of 45 nm. The advantage of shorter pulse duration and controlling the chemical composition of the material by adding of cross-linker will be discussed concerning the achievable minimum feature size.
2.1 Material synthesis, preparation and post-treatment
To compare the influence of the cross-linker on the minimum feature size we used standard Zr-hybrid material with and without added cross-linker. The detail of the material synthesis using sol-gel base technique has previously been described , here we limit ourselves to a brief review. To manufacture photo-polymerizable methacrylate moieties methacryloxypropyl tri-methoxysilane (MAPTMS) and methacrylic acid (MAA) were used. Zirconium n-propoxide (ZPO, 70% in propanol) acts as an inorganic network former. The molecular ratio of MAPTMS to ZPO was set to 5:1 (20%). After the synthesis of above organic/inorganic material, 0.5%(w/w) of 4,4’-Bis(diethylamino)benzophenone and 15%(w/w) of 2-Dipentaerythritol penta-hexa-acrylate was added as a photoinitiator and cross-linker, respectively.
For the experiments the material was prepared by drop-casting onto a cover glass substrate and drying on a hotplate at 100°C for one hour. After the irradiation process all photosensitive material, which was not polymerized, is removed by dipping the sample into 1-propanol with subsequent drying by means of critical point drying technique to avoid the deformation of the residual structures due to surface tension generated during the vaporization of the solvent.
2.2 Experimental setup
Figure 1 shows the schematic illustration of the 2PP experimental setup, wherefore two different laser sources were used for exposure.
On the one hand we used a home‐built non-collinear optical parametric amplifier (NOPA) which delivers up to 420 mW of average output power at 1 MHz while covering a spectral range from 700 nm to 980 nm, supporting Fourier‐limited pulse durations of 5.9 fs . This NOPA is pumped by an amplifier system consisting of a laser oscillator  and a single pass rod-type fiber amplifier . On the other hand we used a commercially available Ti:sapphire laser oscillator (VENTEON | PULSE:ONE) which supports sub‐8 fs pulse durations in a spectrum ranging from 630 nm to 980 nm with a repetition rate of 80 MHz . All relevant parameters of the laser systems are collated in Table 1 .
To preserve the few-cycle pulse durations inside the material, the pulse chirp, changed by the glass material inside the microscopic lens, is pre-compensated by using dispersive double-chirped mirrors (DCMs) and a CaF2 wedge pair. The pulse compression was approved with a SPIDER measurement behind the focusing objective. The focused laser beam transmits through the refractive index matching oil (noil = 1.518), which fills the gap between the objective lens and the surface of the glass plate, into the polymer deposit on the glass. This sample is moved under the objective lens with translation in x-, y- and z-direction. The requested writing of the photo-polymerized structure was realized by computer-controlled linear stages for all three axes (Physik Instrumente M-511.DD in x-direction and M-505.2DG in y- and z-direction). The scanning speed of the laser beam was variable up to 3/50 mm/s with minimum step size of 50/100 nm, respectively. The recording depth was set to approximately 10 μm beyond the surface of the cover glass to ensure aberration‐free structuring conditions inside the material . A camera was mounted above the folding mirror for online monitoring. The line width and height of the fabricated 2PP structures are analyzed from free-hanging lines between wall shape structures by scanning electron microscope (SEM).
3. Experimental results
Writing of lines with feature sizes in sub-100 nm range requires thorough optimization of the processing parameters. To identify the ideal settings, the influence of pulse power and velocity (equivalent to the repetition rate) was observed independently, without changing the other parameters. With optimized conditions all resulting structures made out of the used material, using the two different laser systems, showed a common behavior: the axial cross section has a larger extent than the lateral. This typical behavior is a result of the high NA value of the used microscope objective for focusing.
Concerning the used writing power it should be mentioned that the measurement of the used power for all results within this article was done in front of the microscope objective, because there is no precise standard method to measure the power inside the focal spot, especially if a tightly focusing oil-immersion microscopic lens is used. This difficulty is due to the very small used average power inside the focal spot and the high divergence of the beam. Even calculations from lens transmission couldn’t give precise values since the power losses in microscopic lenses are caused by the microscope aperture, which leads to a relation between net-output power and incident beam shape and size .
All results presented here may include a not further investigated shrinkage during the drying process. More details can be found in [16,21]. Additionally it should be noticed that the results contains the thickness of a thin gold film layer, which is coated for SEM observation. The actual layer thickness can be negligible comparable to the 2PP line width.
3.1 2PP with sub-10 fs pulses from the 1 MHz NOPA and standard material
A home-built MHz NOPA system in combination with the standard Zr-hybrid material is used in the first experiment. The line width and height increase with increasing input power resulting in an aspect ratio (axial / lateral) of the structures of approximately 2 to 3. In Fig. 2 this manner is displayed for a variation of the used writing power. At a fixed velocity of 100 µm/s and a writing power of 2.8 mW (i.e. 2.8 nJ of pulse energy) it was possible to realize a perfect straight line with an axial structure size of 90 nm, whereas in the lateral extent the minimum dimension is approximately 240 nm (see SEM images in Fig. 3 ). With the central wavelength of 850 nm this equals a feature size of ≈λ/10 and ≈λ/4, respectively.
The deviation of the height values for higher writing power from the linear trend (see Fig. 3) is not finally understood as yet. One reason could be an extension of the focus length due to self-focusing. On the other hand, variation of line width fabricated at 0.8 mW of input laser power as a function of scanning velocity indicates also a linear decrease, which reveals structure size influences on the number of irradiated laser pulses.
3.2 2PP with sub-8 fs pulses directly from an 80 MHz oscillator and cross-linker material
The multiple methacrylate groups of the used cross-linker (DPMA) molecules contribute to the photo polymerization process along with the inorganic ZPO, resulting in an enhanced stability and therefore an increase of the survival odds for 2PP structures during the drying process ending up with finer feature sizes. Adding 15% of the cross‐linker mentioned above to the standard material may lead to a fractionally deviation in the refractive index, which differs to that of the standard material (nstd: 1.502) . The exact refractive index of the cross-linker material is still under investigation.
Figure 4 shows the thinnest 2PP line fabricated by the sub-8 fs pulses of the commercial Ti:sapphire oscillator in combination with the cross-linker material. The average power is set to 55 mW, corresponding to ≈0.7 nJ of pulse energy, and 5 mm/s of writing speed. The resulting line width of this material shows also a linear dependency on the power and velocity similar to that of the standard material shown in Fig. 2. The energy threshold of ≈0.7 nJ and 0.8 nJ for the 2PP process at a velocity of 5 mm/s is very similar for both materials and laser systems. It shows the potential to realize structures with an axial structure size of 45 nm and a lateral dimension of approximately 80 nm, resulting in an aspect ratio < 2. With a central wavelength of 800 nm this corresponds to a feature size of >λ/17 and λ/10, respectively. Additionally, Fig. 4 reveals that the SEM electron beam induced a serious deformation of the supporting-structure due to thermal accumulation into the polymerized structure. This is a unique behavior which was never observed in the case of standard Zr-hybrid material. For this reason it was not possible to take a higher number of SEM pictures of the same structure without destroying the supporting-structures and the lines themself. The deformation was permanent and does not regress.
4. Conclusion and outlook
In conclusion we have demonstrated optical and chemical approaches for a reduction of the feature size in 2PP based on few-cycle laser pulses. On the one hand we have obtained an improved resolution by adding cross-linker to the standard sol-gel Zr-hybrid material to tweak the chemical side of the 2PP process resulting in a resolution as high as 45 nm. These results reveal that the addition of cross-linker improves the structural stability and increases the ability of the structure to resist the stress caused during the development process. On the other hand we also showed that the reduction of the used pulse duration to sub-10 fs allows obtaining a resolution enhancement, concerning to the same material machined with ≈50 fs pulses, where 150 nm without and 82.5 nm resolution with cross-linker was achieved . We showed 90 nm resolutions in 2PP process initiated by a 1 MHz NOPA system supplying sub-10 fs pulse durations using the 20% Zr-hybrid material without cross-linker and to our knowledge for the first time 45 nm feature size of a 2PP structure in 20% Zr-hybrid material with added cross-linker produced directly with an 80 MHz sub-8 fs laser oscillator.
As a result of the determined energy threshold of ≈0.7 nJ and the 2PP fabrication close to this, we believe that there is no further potential to decrease the feature size with the used materials and laser systems. For a further improvement of the resolution in 2PP the usage of other techniques like diffractive superresolution elements , spatial light modulators  or high NA hybrid optics  will be promising.
This work was supported by the German DFG-funded priority program (SPP1327) on ‘Optically generated sub-100 nm structures for technical and biomedical applications,’ within the sub-project ‘Development of functional sub-100 nm 3D two-photon polymerization technique and optical characterization methods’ .
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