Robust and reliable actinic ptychographic imaging of highly periodic structures in EUV photomasks

Abstract

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.

Visualizing the ultra-structure of microorganisms using table-top extreme ultraviolet imaging

“…However, in the EUV spectral region, coherent radiation with steadily increasing photon flux and brilliance can be obtained utilizing high harmonic generation (HHG) [24] driven by femtosecond high-average power lasers. Within the past decade, these sources have seen tremendous progress in terms of photon flux and stability [25], which enables nanoscale coherent imaging on a table-top [26] with applications ranging from reflectometry [27] and wavefront sensing [28,29] to material sciences [30,31], especially the imaging and inspection of semiconductors and silicon-based nanomaterials [32][33][34]. EUV radiation provides very high absorptionand phase contrast, moderate penetration depth, and still possesses short enough wavelengths to resolve sub-20 nm structures [32,35].…”

Characterisation of engineered defects in extreme ultraviolet mirror substrates using lab-scale extreme ultraviolet reflection ptychography

“…As for mirrors and masks used in EUVL, an actinic ptychographic inspection at 13.5 nm will reveal the complex wavefront when installed in the industrial environments. EUV ptychography with a 13.5 nm HHG-based light source has already shown applications in imaging or inspecting patterned EUV masks [24,34,49], characterising element components [28] and evaluating EUV pellicle properties [50]. However, HHG will generally have smaller photon flux at higher energy because of its lower conversion efficiency.…”

Ptychographical iterative engine based on pixel binning for improving the computational efficiency

“…Ptychographical iterative engine (PIE) has attracted a great deal of attention in various research fields, such as x-ray imaging [1], electron imaging [2], bio-and semiconductorimaging [3][4][5], and terahertz imaging [6]. In the PIE's configuration, the spot on the object surface overlaps substantially with its neighboring spots, which can increase the robustness and remove the need for a prior knowledge of the object and probe [7,8].…”