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Journal of Micro/Nanopatterning, Materials, and Metrology
VOL. 23 · NO. 4 | October 2024
ISSUES IN PROGRESS
IN PROGRESS
SPIE publishes accepted journal articles as soon as they are approved for publication. Journal issues are considered In Progress until all articles for an issue have been published. Articles published ahead of the completed issue are fully citable.
Extreme ultraviolet (EUV) attenuated phase-shifting masks are complex structures with stringent requirements for manufacturing precision and materials properties, and they have been object of extensive research lately.
Aim
We aim to characterize the optical constants (n and k) and the thickness of the layers in the mask stack with a nondestructive method.
Approach
Using a spectral reflectometry approach with EUV and soft X-ray illumination at various incidence angles, different layer’s properties in a photomask blank can be selectively probed. The optical constants and the thicknesses of the layers can be obtained by fitting a suitable model to the experimental reflectance.
Results
The optical constants of the Pt-W alloy absorber and the thicknesses of the top three layers of the sample stack were accurately characterized.
Conclusions
Stacked layer’s properties can be selectively probed with the instrument (REGINE) we developed. The properties of the topmost layer can be characterized by assessing the probing depth, before investigating deeper layers with a suitable choice of illumination wavelength and angle of incidence.
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In extreme ultraviolet (EUV) photoresist exposure, the primary and secondary electrons drive chemistry rather than the EUV photons themselves. These electrons have a wide range of energies below approximately 80 eV, which are capable of complex network of reactions during exposure.
Aim
To better understand the ability of electrons of different energies within the EUV primary and secondary electron range, we want to characterize and compare the chemistry induced in pure polymer films by direct exposure to electrons.
Approach
Thin films of poly(tert-butyl methacrylate), poly(methyl methacrylate), and poly(4-hydroxystyrene) were exposed to a 20 to 80 eV electron beam. Outgassing during exposure was characterized in-situ using a quadrupole residual gas analyzer. The thickness changes were measured using ellipsometry and chemical bond structure data were collected using Fourier-transform infrared spectroscopy (FTIR) after exposure to compare different exposure conditions.
Results
Poly(4-hydroxystyrene) demonstrated stability during electron exposures. Exposures of the other two materials led to outgassing of protecting groups, the intensity of which decayed in time. Outgassing, FTIR, and thickness loss data exhibited approximately linear relationships to each other.
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Mirror-based and zone plate-based imaging systems are being used in actinic extreme ultraviolet (EUV) reticle review tools. With regard to zone plates, a short working distance is advantageous in terms of the required spectral bandwidth, manufacturability, and potential throughput and imaging performance. Zone plates therefore typically have a short working distance. The industry has adopted the use of an EUV pellicle to protect the photomask. Imaging photomask through-pellicle requires a working distance larger than 2.5 mm. A zone-plate-based EUV mask microscope with a 3-mm working distance has been commissioned at beamline 11.3.2 of the Advanced Light Source. Through-pellicle imaging at an exposure time of 2 s is demonstrated. The instrument achieves an image contrast of 95% on large features on a photomask with a tantalum-based absorber. Imaging down to 45-nm half pitch (mask scale) is demonstrated. A NILS of 2.55 is achieved on 60-nm half-pitch (mask scale) lines and spaces. These results demonstrate that zone-plate-based imaging systems can meet the requirements of an actinic EUV mask review tool in terms of imaging performance and throughput in an instrument compatible with EUV pellicles.
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A key element of semiconductor fabrication is the precise deposition of thin films. Among other aspects, the quality of interfaces between different materials plays a crucial role for the success of further processing steps.
Aim
We here present a combined quantitative study of soft X-ray reflectometry measurements compared to scanning transmission electron microscopy and energy dispersive X-ray spectroscopy (STEM-EDX) on stacked thin film samples of silicon and silicon-germanium (SiGe).
Approach
The thin film structures feature two distinct germanium concentrations in the SiGe layers and are produced for complementary field-effect transistor applications. We use synchrotron-based, angle-, and energy-resolved broadband reflectance to investigate the sharpness of the layer interfaces, which is accessible through rigorous modeling of the acquired data. Complementary, the samples are investigated using STEM-EDX on thin lamellas across the interfaces, which give a direct representation of the interface sharpness through the varying germanium content.
Results
Layer thicknesses and interface properties are studied with the two methods. As a side-product of the measurement, the optical constants of the different SiGe compounds are determined and reported.
Conclusions
We find a very high correlation of the retrieved values between both methods and discuss their comparability and limits.
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Curvilinear masks are coming. With multibeam mask writers in production, leading-edge mask shops now are able to write curvilinear masks in the same mask write times as any Manhattan masks. As Samsung and Luminescent showed long ago, curvilinear mask shapes produce the best wafer process windows.
Aim
The goal of our study is to provide an overview of the current state of the development of curvilinear masks.
Approach
We cover the technical background and motivation for curvilinear masks, and the practical application of curvilinear inverse lithography technology (ILT) masks written by both multibeam and variable-shaped beam mask writers. ILT is a form of optical proximity correction, wherein software computes target mask shapes to optimize wafer quality, both in the nominal lithographic projection conditions being as close to the target wafer shapes as possible, and to minimize the amount of variation in the shape or size due to mask and wafer manufacturing variation. We review the current state of readiness of the mask-making infrastructure for curvilinear masks, including mask rule checking, metrology, inspection, and repair. We also review studies that show that curvilinear masks bias more faithfully to the wafer, and that curvilinear masks are more reliably manufacturable.
Results
Curvilinear masks have been shown to be more manufacturable.
Conclusions
Manufacturable mask shapes are more reliably manufactured. Curvilinear masks improve both mask and wafer variability. We suggest that allowing certain curvilinear targets can make designs more manufacturable and more resilient to manufacturing variation on wafers, while decreasing power, increasing clock speeds, and making designs smaller.
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