We present recent experiments conducted with our tabletop EUV reflectometer to characterize the detailed geometry, composition, and topography of several nanostructured, polymer-based samples. The EUV measurements are performed without sample damage or any significant sample preparation. Correlative STEM measurements confirm the EUV results, but require extensive sample preparation.
As EUV lithography transitions to high volume manufacturing, actinic photomask inspection tools at 13.5 nm wavelength are attractive for understanding the printability of EUV mask defects, as well as for in-fab monitoring for possible defects emerging from extended use. Ptychography is a lensless imaging technique that allows for phase-and-amplitude, aberration-free, high-resolution imaging in the EUV. Moreover, sources based on high harmonic generation (HHG) of ultrafast lasers are a proven viable coherent light source for CDI, with flux sufficient for rapid large-area inspection and small-area imaging. By combining CDI and HHG, we implemented actinic EUV photomask inspection on a low-cost tabletop-scale setup. Moreover, we propose and demonstrate a solution to the decade-long challenge of ptychographic imaging of periodic structures through careful illumination engineering.
We present a tabletop EUV imaging reflectometer that can characterize the geometry and composition of nanostructures. The setup can be operated in multiple modes — intensity reflectometry, scatter-reflectometry, or imaging reflectometry depending on the length scales and periodicity of the nanostructures present. All these modes of metrology can be performed non-destructively and in a lab-scale setup, using a tabletop high-harmonic coherent EUV source.
As EUV lithography transitions to high volume manufacturing, actinic inspection tools at 13.5 nm wavelength are attractive for understanding the printability of EUV mask defects, as well as for in-fab monitoring for possible defects emerging from extended use. Coherent diffractive imaging (CDI) is a lensless imaging technique that allows for phaseand-amplitude, aberration-free, high-resolution imaging in the EUV. Moreover, sources based on high harmonic generation (HHG) of ultrafast lasers are a proven viable coherent light source for CDI, with flux sufficient for rapid large-area inspection and small-area imaging. By combining CDI and HHG, we implemented actinic EUV photomask inspection on a low-cost tabletop-scale setup. Moreover, we propose and demonstrate a solution to the challenge of ptychographic imaging of periodic structures through careful illumination engineering.
We implement an elegant hardware upgrade in our EUV imaging reflectometer to significantly reduce noise from beam power fluctuations. Our instrument non-destructively characterizes the composition and morphology of samples both transversely and depth-resolved, by implementing an EUV coherent diffractive imaging (CDI) microscope in a variable-angle reflection geometry. This upgrade significantly improves signal-to-noise ratio (SNR) by using an EUV beam splitter to simultaneously monitor the beam intensity before and after reflection from the sample. Fluctuations in power may then be normalized out, resulting in greatly improved SNR — an enhancement of ~6x in our first, non-optimized implementation.
Coherent Fourier scatterometry (CFS) via laser beams with a Gaussian spatial profile is routinely used as an in-line inspection tool to detect defects on, for example, lithographic substrates, masks, reticles, and wafers. New metrology techniques that enable high-throughput, high-sensitivity, and in-line inspection are critically in need for next-generation high-volume manufacturing including those based on extreme ultraviolet (EUV) lithography. Here, a set of novel defect inspection techniques are proposed and investigated numerically [Wang et al., Opt. Express 29, 3342 (2021)], which are based on bright-field CFS using coherent beams that carry orbital angular momentum (OAM). One of our proposed methods, the differential OAM CFS, is particularly unique because it does not require a pre-established database for comparison in the case of regularly patterned structures with reflection symmetry such as 1D and 2D grating structures. We studied the performance of these metrology techniques on both amplitude and phase defects. We demonstrated their superior advantages, which shows up to an order of magnitude higher in signal-to-noise ratio over the conventional Gaussian beam CFS. These metrology techniques will enable higher sensitivity and robustness for in-line nanoscale defect inspection. In general, our concept could benefit EUV and x-ray scatterometry as well.
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