Imaging of nanostructures with sub-100 nm spatial resolution using a desktop EUV microscope

The EUV microscope was equipped with an ellipsoidal mirror with Mo/Si coating to focus extreme ultraviolet (EUV) radiation onto an object. A Fresnel zone plate (FZP) objective was used to form a magnified image onto a EUV-sensitive CCD camera in the transmission mode. The use of the gas-puff target eliminates the debris production problem associated with solid targets. Quasi-monochromatic EUV radiation, which is required for the use of Fresnel optics, was produced by spectral selection of a single line emitting at 13.8 nm wavelength from argon plasma. So far, different test objects (samples) were imaged with this setup: Cu mesh with thickness of ~4 μm [4] and carbon foil, 70 nm thick, with holes [5], where the EUV images have been obtained with the half-pitch spatial resolution approaching ~50 nm (3.7λ) in a very compact setup. The scheme and experimental arrangement are shown in Fig. 1a) and a photograph of the EUV microscope is shown in Fig. 1b).

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The laser plasma EUV source, used in the experiment, has been developed for EUV metrology applications in the frame of the MEDEA+ project [13] and later modified for quasi-monochromatic emission in the 13–14 nm wavelength range, described in detail elsewhere [14]. This source has the advantage over other compact sources that simply by choosing a suitable gaseous target it is possible to change both the peak emission wavelength and the inverse relative bandwidth of the emission from the plasma. Moreover, the gaseous target does not produce any debris associated with solid targets.

Ar plasma was produced by Nd:YAG laser (Eksma) irradiation of a gaseous target with pulse duration of 4 ns, wavelength 1,064 nm, and energy 0.74 J. The plasma radiates in a very broad range of wavelengths, dominantly in the EUV range (5–50 nm wavelength). By using additional spectral filtering, it is possible to shape the spectral emission of the source. The source can operate at up to 10 Hz repetition rate. A pressure of 2 × 10⁻³ mbar was constantly maintained in the chamber during the source operation.

EUV radiation from the plasma was both focused and spectrally filtered by a Mo/Si multilayer ellipsoidal mirror (condenser) with 80 mm in diameter. The mirror was developed in collaboration with Reflex s.r.o. (mirror substrate) and Fraunhofer Institut für Angewandte Optik und Feinmechanik (coating). The multilayer coating was optimized for 13.5 ± 0.5 nm (FWHM) wavelength range and 45 degrees incidence angle. The theoretical reflectivity of the mirror is 37.7 % at 13.5 nm wavelength for an unpolarized light.

The laser plasma source was optimized for efficient generation of EUV radiation from Ar plasma. The in band (λ = 13–14 nm) photon flux was (8.8 ± 0.5) × 10¹⁰ photons per pulse in a horizontally elongated spot with a size 1.09 × 0.39 mm² (FWHM) [14]. That corresponds to the energy of 1.29 μJ/pulse.

To eliminate longer wavelengths (λ > 18 nm), a 100 nm thick, 10 mm diameter, free-standing Zr filter (Lebow) was used. The Zr filter was positioned ~4–5 mm upstream the object. A quasi-monochromatic spectrum was obtained from Ar plasma after spectral filtration by the condenser. The resulting spectrum has two main peaks at 13.793 and 13.844 nm wavelength—transitions in Ar^VIII [15]. The inverse relative bandwidth (FWHM) of the filtered spectrum is λ/Δλ = 140 at λ = 13.84 nm.

As an object, we used NaCl crystallized on a 15-nm thick non-porous silicon membrane (SPI Supplies) with dimensions 0.07 × 1.5 mm². The substrate—a silicon wafer 3 × 3 mm² in size and 100 μm thick—contains two rectangular shape membrane windows. The NaCl crystals were prepared from phosphate buffered saline (PBS) solution (Lonza, Cat.No. BE17-516Q, contents KH₂PO₄ 144 mg/L, NaCl 9000 mg/L, Na₂HPO₄ 795 mg/L). 5 µl of the solution was deposited onto the membrane. The water from the solution was evaporated at the temperature of 21 °C and atmospheric pressure, resulting in re-crystallization of NaCl. During the process of re-crystallization, stress was introduced to the membrane causing random cracks of various sizes to appear in the NaCl crystals and the silicon membrane. Optical images of the object obtained with 40× (NA = 0.65) and 100× (NA ~ 1) objectives are shown in Fig. 2a and b, respectively.

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The object, placed 254 mm from the condenser mirror, is imaged using a FZP objective (Zone Plates Ltd.) onto a EUV sensitive CCD camera iKon-M (Andor) with 1,024 × 1,024 pixels and 13 × 13 μm² pixel size. The diameter of the FZP is D = 200 μm, number of zones N _FZP = 1,000 and an outer zone width Δr = 50 nm [The ZP was fabricated by Zone Plates Ltd., using electron beam lithography in 220 nm thick PMMA (polymethyl methacrylate) layer spin- coated on top of a 50 nm thick silicon nitride Si₃N₄ membrane]. The FZP was fabricated and optimized for λ = 13.5 nm. In the case of quasi-monochromatic illumination at λ = 13.84 nm from Ar plasma, the focal length of the FZP is f = 722.5 μm and the numerical aperture NA = 0.138. The depth of focus (DOF), calculated as DOF = ±λ/(2NA ² ) is equal to ±363.4 nm. Typically, the magnification is equal to 520× (FZP magnification), adjusted by changing the camera-to-FZP distance and refocusing. This results in 25.9 × 25.9 nm² pixel size and the field of view 26.4 × 26.4 μm². More information regarding the EUV microscope can be found in [4, 5]. Geometrical numerical apertures of the condenser mirror in the horizontal and vertical directions are equal to NA_cH = 0.11 and NA_cV = 0.15, respectively. They are similar to the numerical aperture of the FZP, NA_FZP = 0.138. Since \( {{\upsigma}}_{\text{H,V}} = {\text{NA}}_{\text{cH,V}} /{\text{NA}}_{\text{FZP}} \) are equal to \( \left[ {{{\upsigma}}_{\text{H}} , {{\upsigma}}_{\text{V}} } \right] = \left[ { 0. 8 , 1. 1} \right] \), the sample is incoherently illuminated [16]. The FZP was mounted on a three axis translation stage driven by vacuum compatible step motor actuators (Standa). A piezoelectric single-axis flexure stage (Thorlabs, model NF15A) with 25 μm travel was used to adjust precisely the object-to-FZP distance at a rate of 3 V/μm, to locate the image plane with position accuracy better than the DOF. In our case, the theoretical resolution of the piezo-stage was ~10 nm ≪ DOF. A circular beam block with 12 mm in diameter was placed ~15 cm from the FZP to achieve a conical illumination of the object and avoid stray light through the FZP.

In the case of “binary” transmission, test objects such as Cu mesh, 25–50 EUV pulses at 2 Hz repetition rate were necessary to obtain a single image [5]. The required exposure for imaging of NaCl crystals deposited on the silicon membrane was 200 EUV pulses at 2 Hz. The source can operate at up to 10 Hz repetition rate; however, the pressure buildup in the microscope chamber might cause re-absorption of EUV photons in neutral gas. Therefore, the lower repetition rate was used for image acquisition. The CCD camera was cooled down to −20 °C to decrease intrinsic noise during the image acquisition.

For the purpose of comparison, the same area of the sample was imaged with a SEM microscope (Tescan VEGA II SBU) and with the EUV microscope described above. SEM image was obtained after deposition of ~10 nm of Au to avoid charging during image acquisition at 30 kV at magnification of ~4,000× (sample to screen magnification).

From Fig. 2, it is clear that the cracks in the thin silicon membrane are usually quite large, in the range of one to tens of microns. Crystal film, however, has also a variable transmission to the visible light, but except that, nothing else can be concluded about the object. To obtain more information, the same object was imaged using SEM and EUV microscopes.

The SEM image of the membrane with NaCl crystals on top is shown in Fig. 3a. The same region was also imaged with the EUV microscope. To observe the same area, EUV image was produced by stitching of 16 partially overlapping images in a 4 × 4 array, which can be seen in Fig. 3b. Each image was obtained by precise adjustment of the object-to-ZP distance, using a previously mentioned piezo actuator, with position accuracy of 300 nm, smaller than the DOF. It was necessary to satisfy this condition, because otherwise the EUV image will be out of focus and will have lower spatial resolution. The area of both SEM and EUV images is approximately 50 × 50 μm². The same region is also indicated in Fig. 2 with dashed boxes. The SEM image in Fig. 3a shows NaCl crystals in detail. Their complex structure obscures, however, the information about the membrane itself. In the SEM image, it is possible to see dents, cracks, and pits in the sample, but in some cases, it is not possible to determine how deep they are.

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The EUV mosaic shown in Fig. 3b, on the other hand, does not reveal any surface features formed by the thick layer of crystallized salt. The crystals are several microns thick and their transmission at EUV wavelength of 13.8 nm is ~1.5 × 10⁻⁷ (Assumed NaCl thickness was 1 µm, using CXRO database, http://henke.lbl.gov/optical_constants/filter2.html). However, the EUV image shows regions with thickness smaller than ~100 nm that are represented by the bright regions. These are formed by very deep gaps in the NaCl layer and small cracks and holes in the supporting Si membrane. Intensity of these regions is related to their transmission and, as can be seen in Fig. 3b, some regions have higher transmission than the others. Despite the images are rather noisy, they confirm that EUV microscopy can be used, for example, in thin film studies with several advantages. Figure 3 shows that the EUV radiation used for image acquisition enhances some information about the object which is also present in the SEM image, however, more difficult to extract. The brightest regions indicate cracks in the supporting membrane, as can be observed comparing images of a 2–8 μm wide vertical crack, shown in top-right region of a dashed box in Fig. 2b and on the right in Fig. 3b. Other, less intense features are also visible suggesting lower object transmission and, in turn, higher thickness.

Another interesting fact is that there are some features, which are marked with white arrows that are visible in both SEM and EUV images presented in Fig. 4a, b. However, there are also some cracks in the crystal film, visible in the SEM image, such as one marked with dotted arrow, but not present in the EUV image, as can be seen in Fig. 4c,d. Again, on the left SEM images are shown and on the right are corresponding EUV images of exactly the same regions of the sample. Interestingly, some features, such as the cracks in Fig. 4a and in Fig. 4d, are almost indistinguishable in the SEM images. Corresponding EUV images, on the other hand, show completely different depth of these features. One of them, shown in Fig. 4a, is undoubtedly all the way to the supporting membrane, while another crack in Fig. 4d is only a near-surface feature. Thus, in this case, the EUV radiation enables additional, complementary information about the object to be obtained, which is based on an intrinsic property of EUV light, and that cannot be obtained directly using SEM microscopy or visible light microscopy.

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Images obtained with test samples generally exhibit higher signal-to-noise ratio (S/N) than images of non-trivial samples. Moreover, the EUV source photon flux is often insufficient. In addition, the sample, used in this study, although is suitable to show the advantages of using EUV light, however, is difficult to image, because the only signal are the photons passing through small, sometimes around 100-nm wide, cracks in the membrane. These photons produce an actual image on the CCD camera. This is the reason, why the S/N ratio is not high. Despite that, some nanoscopic features can be recognized in the EUV images.

Typical intensity cross-sections throughout some small features in the EUV images are presented in Fig. 5. The cross-section shown in Fig. 5a, obtained without spatial averaging, clearly indicates the signal associated with a feature ~260 nm wide. Moreover, as can be seen in Fig. 5b, features with characteristic size as small as 4 pixels can be resolved. For the given magnification and pixel size of 25.9 nm square, the FWHM width of this small feature approaches 100 nm with S/N ratio of 2.2.

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The smallest features, of this non-trivial sample, were observed with spatial resolution that was not measured directly using knife-edge test. It is due to the fact, that this sample does not have a proper edge, required by the knife-edge resolution test. However, we can say that the obtained resolution is comparable to the resolution measured with test samples. The half-pitch spatial resolution, reported previously using test samples, was ~50 nm, while the smallest visible feature width is approximately twice that value, ~100 nm, corresponding to a full pitch spatial resolution, or Rayleigh resolution. The theoretical resolution for the microscope system, taking into account the ZP outerzone width of 50 nm, is limited by the zone plate objective to half-pitch spatial resolution equal to ~30 nm, for monochromatic illumination. For actual, quasi-monochromatic illumination EUV microscope half-pitch spatial resolution was previously, assessed with test samples and reported to be ~50 nm [5].