In recent years, technological progress in nanotechnology has pushed structure sizes to its limits. As an example, in the semiconductor industry, structures well below 100 nm are routinely produced. The characterization of such structures is a demanding and very important task. Classical microscopy methods do not allow direct imaging in this regime because of the Abbe diffraction limit. Nevertheless, characterization of sub-wavelength structures in the far-field is possible using interferometric Fourier transform scatterometry (IFTS) combined with numerical simulation in a feedback loop. Here, we show that the resolution of this method can be considerably enhanced by use of additional plasmonic nanoantennae structures which transform scattering near-field information into the optical far-field. These structures were realized using different photofabrication approaches.

4.1. Introduction

An electromagnetic near-field around metallic nanostructures is associated with the excitation of localized surface plasmons. The realization of plasmonic near-field to far-field transformers in the optical region between 400 and 800 nm requires structures of sub-wavelength dimensions with a pronounced optical near-field and a strong light scattering ability. When the size of the objects is smaller than the optical wave-length λ, the structures start to scatter the localized plasmonic excitations into the far-field. The enhanced information, now available in the far-field, enables detection of structural feature sizes of sub-wavelength critical dimensions with optical methods. The principle idea of this sub-wavelength far-field characterization of groove and grating structures, providing nanometric resolution, is shown in Fig. 4.1 (a). Additional light scattering from optical near-fields, carrying information about sub-wavelength structural features, into the far-field is accomplished by rod-like nanoantennae.

Fig. 4.1

Principle idea of near-field to far-field transforming plasmonic structures (a). Schematics structures with added plasmonic features as a plasmonic grating (c) or scattering near-field nanoantennae (b).

Fabrication and far-field characterization of sub-wavelength grating structures are presented in this work. Measurements of the scattered light with and without rod nanoantennae from gold-covered samples have been performed and compared with numerical results. Sensitivities to structural dimension of 50 nm have been obtained, using light with a wavelength of 632.8 nm. The fabrication approach for both the sub-wavelength grating structures and the near-to-far-field (NTFF) transformation nanoantennae is based on high-resolution two-photon polymerization (2PP). Repeatable realization of line and dot structures with dimensions down to 50 nm is presented.

4.2. Fabrication approaches for near- to far-field transformation optics

For a demonstration of enhanced optical imaging below the diffraction limit, high-resolution grating structures (see Fig. 4.1 (b & c)), consisting of periodic sequences of lines of different widths have been fabricated with and without NTFF structures. These structures are described in detail in Section 4.3. Examples of test structures with minimum line widths down to 230 nm fabricated in a positive tone photo resist S1805 by microscopic projection photolithography (MPP) [1] are shown in Fig. 4.2.

Fig. 4.2

Structures under consideration for near-field to far-field transformation of plasmonic structures. Upper left: grating structure with sub-wavelength ridge widths of 350 nm fabricated by MPP. Upper right: grating with polymer pillars between the grating (more...)

The NTFF transforming structures consist of nanoantennas with dimensions of less than 100–300 nm. In order to achieve high flexibility in the realization of the structures and high resolutions down to the sub-100 nm range for both the fabrication of grating NTFF structures, the process of two-photon polymerization (2PP) was applied. Especially for the creation of high aspect ratio NTFF structures, the potential of real 3D structuring using 2PP was exploited.

4.2.2. 3D two-photon polymerization with additional crosslinker

The influence of the crosslinker dipentaerythritol penta-methacrylate (DPMA) on the achievable dimensions of supported isolated polymer lines was investigated in the experiments.

4.2.2.1. Materials

A zirconium-based inorganic-organic hybrid polymer (Zr-Hypo) was used as basis material for the investigations. The material has been demonstrated to exhibit only minor volume shrinkage, which is favorable for high resolution structuring [34]. Synthesis of the zirconium-based inorganic-organic hybrid polymer was carried out by a sol-gel-process [35]. The commonly available radical photoinitiator 4,4′-bis (dimethylamino) benzophenone (Bis) was used for initiation of the polymerization process. A diagram of the synthesis protocol is shown in Fig. 4.3.

Fig. 4.3

Synthesis of zirconium-based inorganic-organic hybrid polymer.

Methacryloxypropyltrimethoxysilane (MAPTMS, Sigma-Aldrich) was hydrolyzed with 0.1 M hydrochloric acid (HCl, Sigma-Aldrich). Methacrylic acid and zirconium isopropoxide were mixed separately in a molar ratio of 4:1 and stirred for 30 min.

The chelated zirconium isopropoxide was subsequently added to the hydrolyzed silane precursor, and a small amount of water added. The molecular ratio of MAPTMS to ZPO was set to 5:1 (20 %). The mixture was stirred for another 30 min. At the end, 0.5 wt % of the photoinitiator was added. Before structuring, the pre-polymer was drop-casted onto a microscope cover glass slide with a thickness of 150 µm and heated for 1 h to 100 °C. A mixture of 50 vol % isopropanol (Iso, Sigma Aldrich) and 50 vol % 4-methyl-2-pentanone (methyl isobutylketone, MIBK, Sigma Aldrich) was used to develop the realized structures, i.e. to dissolve the non-polymerized portion of the material. The samples were immersed in this mixture for 1 h. Line widths in woodpile crystal structures of 320 nm have been demonstrated with this material [34].

The performance of this material was compared to the same material with an addition of 15% crosslinker DPMA for the realization of sub-100 nm structures. The simple change in the synthesis protocol is shown in Fig. 4.4. The amount of DPMA was added to the basis material described above just before addition of the photoinitiator, which again was 0.5 wt % Bis.

Fig. 4.4

Synthesis of DPMA-amplified Zr-Hypo.

2PP of these two materials was initiated using the Ti:Sapphire based high-peak power femtosecond laser oscillator Femtosource scientific XL500 (Femtolasers) with a repetition rate of 5 MHz, a central wavelength of 800 nm, and a pulse width smaller than 50 fs. The laser pulses were spatially focused into the bulk volume of the photosensitive material by a microscope objective lens. A 100× immersion oil microscope objective with a numerical aperture of 1.4 was used to obtain the best focus. The laser beam was focused from the back of the cover glass plate. In order to generate the test structures, the focus was scanned through the polymer volume by moving the sample with a computer controlled positioning system (Physik Instrumente PI, 3x M-126.DG for x-, y-, and z-axes). The scanning speed of the laser beam was kept at 1 mm/s. The laser beam focusing in this configuration is fixed.

For the development of the realized structures, i.e. to dissolve the non-polymerized portion of the material, a mixture of 50 vol % isopropanol (Iso, Sigma Aldrich) with 50 vol % 4-methyl-2-pentanone (methyl isobutylketone, MIBK, Sigma Aldrich) was used. The samples were immersed in this mixture for 1 h.

4.2.2.2. Experimental results

The writing speed was fixed at 1 mm/s. The applied average laser power varied within the range of 0.1–10 mW. The line widths and heights were measured depending on the applied average laser power using high resolution SEM images. The line widths and heights measured for the two materials Zr-Hypo and DPMA-Zr-Hypo are shown in Fig. 4.5. Note that the applied average laser power in the case of the non-amplified Zr-Hypo has to be multiplied by a factor of 10. At first glance it is obvious that there is an abrupt cut-off for the achievable widths and heights towards low laser powers in all cases. This cut-off is due to the stability of the materials. Below a certain laser power, and thus a certain achieved polymerized material strength, the structures do not withstand the development process. The structures are destroyed by the evaporating solvent mixture. Further, it can be seen that the smallest lines can indeed be achieved with amplified DPMA-Zr-Hypo for a minimum applicable average laser power of 0.5 mW, whereas the smallest lines were realized for the Zr-Hypo for an average laser power of 1 mW.

Fig. 4.5

Supported line widths (left) and heights (right) achieved for the two polymers Zr-Hypo (diamonds) and DPMA-Zr-Hypo (circles).

The results and the achieved minimum line widths and heights for Zr-Hypo and DPMA-Zr-Hypo are summarized in Fig. 4.6. In the non-amplified material, the smallest reproducible line width achieved was around 150 nm. Smaller polymerized structures have principally been observed in the online process observation, but were washed away during development.

Fig. 4.6

Summary of the structuring of Zr-Hypo and amplified DPMA-Zr-Hypo. Lines of less than 82.5 nm in width were reproducibly fabricated under the influence of an additional crosslinker.

The reproducible smallest line width achieved in the amplified polymer containing 15 % DPMA was measured to be 82.5 nm according to the SEM images. It should be noted that the structures had been isotropically covered with a gold layer of at least 10 nm thick by sputtering in a gas discharge with a background gas pressure of 10 mbar for SEM inspection [36]. Under these conditions the achieved minimum line width can be estimated to be smaller than 62 nm. In further experiments, free-standing polymer structures with high aspect ratios of 10:1 were also obtained. The smallest diameter of a rod-like nanoantenna has been measured to be 40 nm.

4.2.3. 2D two-photon polymerization of E-Shell 300 on dielectric surfaces

2PP has also been investigated with materials which are cheap and easily available commercially. One of those materials is E-Shell 300, which is commonly used for 3D printing and hearing aid fabrication. The material has a high flexural strength up to 88.4 MPa and is suitable for the fabrication of structures with dimensions below 100 nm. The photoinitiator used in the commercial product is Irgacure 127, at a concentration of 5 wt %. Initial experiments have shown that the material can directly be structured by 2PP using frequency doubled ytterbium:potassium-gadolinium-tungstate laser radiation with 200 fs pulse duration. A laser power variation was performed to quantify the material properties and the structural sizes obtainable with 2PP. The exposure time for the fabrication of single voxels was fixed to 20 ms. A typical result is shown in Fig. 4.7. In the first test of the material variable voxel sizes ranging from 200 nm down to 50 nm were obtained.

Fig. 4.7

2PP with E-Shell 300: fabrication of voxels for material characterization. Upper left: large array of pillar structures. Upper right: power variation for a fixed exposure time of 20 ms. Minimum voxel diameters of 53 nm have been measured. A grating structure (more...)

Line structures have also been fabricated with the same material. Gratings consisting of polymer lines with diameters of 100 nm and below have been realized, as demonstrated in Fig. 4.7. The writing speed was set to 10 µm/s and the laser power adjusted to 2 mW.

By carefully adjusting the laser focus with respect to the substrate surface, line widths of 58 nm down to 50 nm were obtained for applied laser powers of 1 mW, and slow scanning speeds of 5 µm/s and 7 µm/s respectively. At these dimensions, even very small fluctuations in the laser beam power and the positioning stages have huge influences on the structuring results. These fluctuations are slightly visible in the images in Fig. 4.7. They have no influence on structures with a critical dimension greater than 100 nm however.

Gratings with and without NTFF nanoantennae have been realized from E-Shell 300. The results are shown in Fig. 4.8. The period of the grating was chosen to be 450 nm. A line width of 95 nm and a height of 70 nm for the grating ridges were obtained for a scanning speed of 7 µm/s and a laser power of 2 mW in front of the focusing microscope objective. The NTFF nanoantennae with 500 nm period are cylindrical pillars with a rounded top of 110 nm diameter and a height of 150 nm.

Fig. 4.8

Grating and NTFF structures fabricated by high-resolution 2PP. Upper left: overview image of an exemplary 20 µm × 20 µm grating with nanorod antennae. Upper right: magnified image showing the regularity of the structure. Bottom (more...)

4.3. Fourier scatterometry on gratings with designed near-field structures

The characterization of sub-lambda structures with a fast non-destructive optical method is a common metrology task.

One of the most widely used techniques for these tasks is the scatterometry method [37]. Scatterometry is a method for analysis of the diffraction spectrum from a periodic arrangement of nanostructures. It is used to reconstruct the unknown structure parameters by comparison of measured and simulated spectra, which is why it belongs to the field of so-called model-based metrology methods [38].

There have been various attempts to improve scatterometric sensitivity, mainly by increasing the illumination wavelength range used, variation of the incidence angle or a high numerical aperture illumination, as well as combinations with other measurement methods, such as for example white light interferometry [38]. While all of these methods increase the sensitivity of the measurements by increasing the information content of the measurement itself, a different approach was taken in the work presented. The simulation branch needed for all scatterometric measurements can itself be exploited to design an optimized scatterometric sensitivity inherent to the structure. While this may not be generally applicable due to practical constraints, it is conceivable for alignment, testing or calibration targets.

While we have already shown some promising results based on simulations [39], we now want to verify these results with experimental data. We apply the so-called Fourier scatterometry method [38] to detect the backscattered light of polymer test line gratings fabricated as described in Section 4.2. We then analyze the influence of added near-field structures on sensitivity to line width. In contrast to the simulation study mentioned, we used a brightfield illumination with a broad range of incident directions containing much more information. In addition to new adapted simulations we show a first experimental implementation and verification of the result obtained previously.

4.3.2. Experimental setup

We built a Fourier scatterometry setup which will be explained in the following section. It was completely redesigned and based on a setup used in previous works [39]

We start with the illumination path. A red LED with a wavelength of 617 nm is used as light source. We use a microscope objective (10×) to magnify and depict the end of the polished fiber on the back focal plane of the microscope objective used to illuminate the sample. A polarizer allows selection of s or p polarized illumination. The image of the fiber covers the complete aperture of the backfocal plane of the objective, producing a homogenous intensity distribution. We use a microscope objective with a NA of 0.95 (250× magnification); this means that the sample is illuminated with plane waves covering the angles from 0 to 72°. The backscattered light from the analysed sample is again collected by the same objective, as our microscope works in reflection mode. A beamsplitter is used to image the backfocal plane of the objective and the image plane of the sample at the same time. The backfocal plane is imaged with the help of a Bertrand lens on a CCD camera, while the image plane is imaged with help of a matched tube lens. Both lenses are specially designed and aberration -corrected for the microscope objective used.

Besides the illumination and imaging path of the Fourier scatterometry microscope, a reference arm for a Linnik type setup is also included in the setup. It includes a matched objective with identical NA as the imaging objective and allows imaging of the interfering backfocal planes of both objectives. This type of measurement is detailed in [39] but will not be used in this setup. The reference path is blocked with an absorbing plate. The CAD construction of the setup as well as the schematic setup showing the detailed light paths can be found in Fig. 4.9.

Fig. 4.9

CAD design of the setup (top) and scheme of the experimental setup showing the different beam paths (bottom).

A typical measurement consists of searching the grating to be analyzed on the sample using classical imaging microscopy and focusing directly on top of the structure and then recording the pupil plane images for both polarizations (s and p). Using structures which only vary in width and comparing the images allows extraction of the sensitivity of the signal towards changes of the line width.

4.4. Experimental results

First the structures are characterized with an electron microscope to get an impression of the different structure parameters. The images are shown in Section 4.2. This data is used for the modelling of the structure to be used by the simulations. We define the sensitivity towards a parameter as the change in the pupil plane intensity compared to the absolute changes of the parameter.

The analyzed structures consist of polymer test line gratings fabricated as described in Section 4.2. The period of the lines is 1800 nm, while the height is 500 nm. There are gratings with different line widths (250 and 350 nm). The complete grating is covered with a 20 nm layer of gold.

Pupil images for these structures were taken for both s- and p-polarization. The resulting images and the difference in pupil plane intensity can be found in Fig. 4.10 together with the results of the same measurements and simulations with added nanorods between the lines. In the case with rods, the linewidths are 380 nm and 420 nm, meaning that the difference is smaller than for the lines without the rods (250 to 350 nm). The rod itself has a radius of approx. 175 nm and a height of 500 nm. Again the structures are covered with 20 nm of gold.

Fig. 4.10

Resulting pupil images (simulated and measured) for s- and p-polarization for gratings with and without nanorods between the lines, as well as the corresponding differences when the CD of the gratings is varied. For the lines without nanorods the compared (more...)

Differences between measured and simulated pupil images can have different reasons. The most important is the simplified simulation model which had to be used due to the very large calculation times. The grating was modelled with perfectly steep walls; neither roundings nor line edge roughness was taken into account. The SEM pictures and microscope pictures show that this is a very rough assumption. Additionally, till now we have not taken any aberrations of the optical elements used in the setup into account. The LED light source is modelled as perfectly homogenous and with a very narrow band of only 1 nm wavelength range, while realistically 20 nm should be assumed. Each wavelength would need one simulation and the global simulation time for that would increase drastically. Additionally, the agreement between simulation and experiment is worse in the case of added nanorods. This could indicate that a more sophisticated modelling of the nanorods is needed, which would lead to significantly increased computation time.

The measured and simulated results show comparable absolute differences in pupil intensity for the gratings both with and without rods. It must be noted, however, that the line widths analyzed do not agree for both cases. In the case of the lines without rods, the line width differs from approx. 250 to 350 nm, a difference of 100 nm. While the widths for the lines with rods are much closer; 380 compared to 420 nm, a difference of only 40 nm. The results show almost identical differences in intensity. For that the system is much more sensitive for the case of a grating with rods. This proves the improved sensitivity obtained by adding scattering near-field structures.

4.5. Conclusions

Based on the assumption of higher scatterometric sensitivity of structures with added near- to far-field transformers designed by previous simulation-based studies, we have now provided a first experimental verification for measuring critical dimensions of gratings with resolutions as low as 50 nm. 2PP methods have been developed, improved, and successfully applied for the fabrication of sub-wavelength structures. The experimental results supported by corresponding rigorous simulations show the expected sensitivity gain in the scatterometric signatures.

Acknowledgments

The authors acknowledge contributing work by Tim Fischer and Ayman El-Tamer for the preparation of 3D sub-100 nm structures.

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