Ptychography with table-top EUV light sources

Abstract

The objective of this report is to explore the applications of ptychography with table-top EUV light sources, including a high harmonic generation (HHG, 29.6 nm) and a discharge produced plasma (DPP, 13.5 nm) light source. The applications include characterising defects in EUV lithography (EUVL) mask with reflection-mode ptychography and imaging various transparent samples with transmission-mode ptychography. The correction of diffraction distortion caused by the tilted illumination is considered in our experiments. Ptychography is a technique that uses the measured diffractions from a coherently-illuminated sample to recover the complex functions of both the probe P, which is the illumination spot used to scan the sample, and the object O, which contains phase shift and amplitude changes introduced by the sample. In our data analysis, extended ptychographical iterative engine (ePIE) and difference map (DM) are used for ptychographic reconstruction. Other advanced algorithms like state mixtures and orthogonal probe relaxation ptychography (OPRP) are also used to consider the light incoherence and instability. Multilayer mirrors (MLMs) are widely used as reflection optics in EUV and soft X-ray spectral range. Defect detection and removal are crucial in order to guarantee the quality of MLM. The mask (or reticle) in EUVL is built upon MLMs. Inhomogeneities in the amplitude or phase of the reflected field caused by defects can produce critical errors in the lithographic image of the mask pattern. Reflection-mode ptychography using EUV light is a promising EUVL mask characterisation technique. The most important sample we used was a Si/Mo MLM with programmed defects on the Si substrate. Defects patterns were etched by e-beam lithography and subsequent reactive ion etching on the substrate, forming pits or bumps. These defects on the substrate will cause distortions in layered structures and possible roughness on the surface as well. Besides inspecting the features on the surface, reflection-mode ptychography is also able to provide structural information within the penetration depth of EUV into the MLM sample. In our case, it will be > 50 nm for 29.6 nm light from HHG (angle of incidence, AOI = 45◦) and > 280 nm for 13.5 nm light from DPP (AOI = 6◦). Two different light sources are used: For majority of the research, we used a HHG light source in University of Southampton. It is driven by a Ti-sapphire laser system with a regenerative amplifier, central wavelength of 800 nm, repetition rate of 1 kHz, up to 3 mJ per pulse and pulse length of ∼ 50 fs. 80 mbar argon gas was the HHG medium. The 27th harmonic (29.6 nm) was filtered out by an Al filter and a MLM for ptychographic imaging. We built a setup which was easily convertible between the transmission mode and the reflection mode. A new aperture design was introduced to achieve a bigger collection numeric aperture (NA up to 0.7 for AOI = 45◦) and lower parasitic reflection. In Forschungszentrum J ulich, Germany, we used an xenon gas based DPP light ¨ source. 13.5 nm was filtered out to do imaging, which is the same wavelength used in the industrial EUVL. DPP light source has inherently poor coherence. With the help of a Zr filter, several MLMs and a ¬= 10 µm pinhole, both spatial and temporal coherence were improved to meet the requirements of ptychography. In this report, we will demonstrate the imaging capability of ptychography in three different geometries: conventional transmission mode (CTM), tilted transmission mode (TTM) and tilted reflection mode (TRM). The ideas were first tested on a Helium-Neon (HeNe) laser to verify the feasibility of diffraction correction. A resolution test target by Thorlabs and a silicon wafer with etched features were used as the samples. The three modes were then verified with EUV light sources. With EUV light, polymethyl methacrylate spheres deposited on a 50 nm thick silicon nitride membrane were used as the sample in CTM and TTM. A shadow effect caused by tilted illumination was observed by comparing their reconstructed images. The MLM sample with phase defects mentioned above was used as a sample in TRM. We have achieved a theoretical pixel size of 60 nm (NA = 0.22) and a vertical resolution

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Identifiers

ORCID for Haoyan Lu: orcid.org/0000-0001-5135-3920

ORCID for Bill Brocklesby: orcid.org/0000-0002-2123-6712

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