Photoresist metrics such as resolution, roughness, CD uniformity, and overall process window are often aimed to realize the full potential of EUV lithography. From the view of the materials supplier, improvements over the aforementioned metrics can be achieved by optimizing the functional materials used under the resist. The underlayers can significantly enhance the resist performance by providing appropriate adhesive forces to the patterned features, counterbalancing the tendency of pattern collapse, and improving the resist/substrate compatibility. In this study, we introduce spin-on-glass (SOG) underlayers that can provide adhesion to resist through different mechanisms, including covalent bonding, hydrogen bonding, and Van der Waals force. The underlayers were characterized in depth to understand their coating quality, adhesion to resist, and surface energy. The EUV lithographic performance was evaluated by applying chemically amplified resists (CARs) directly onto these SOGs for line/space features at the target pitch sizes of 30 nm and 28 nm. The lithographic evaluation indicates that the patterning performance strongly depends on resist/underlayer adhesion. By appropriately adjusting the resist/underlayer adhesion, we can realize 28 nm pitch printing with defect-free depth of focus larger than 300 nm and unbiased line-width roughness around 2.2 nm. The plasma etch rate of the SOGs were also evaluated to assess their pattern transfer performance.
The combination photoresist-underlayer fulfills a central role in EUVL for patterning. Moreover, future high numerical aperture (NA) and tight pitches will require very thin layers in the lithography stack, which will increase the impact of chemical interactions at the photoresist-underlayer interface. Adhesion between these layers will be critical to overcome pattern collapse at high aspect ratios whereas at the same time the impact of interfacial effects, such as intermixing, on the lithography performance must be minimized. A fine balance must be found between several chemical interactions, which is a complex exercise with many unknown parameters. In this paper we present how the polar and dispersive components of the surface energy can be used to optimize EUV underlayers in order to achieve the best lithography performance. Subtle changes in the underlayer composition can have a large effect on the lithography performance. A PTD CAR photoresists test case will be used to demonstrate that the photoresist profile, scum formation, line width roughness, pattern collapse and stochastic defects can all be tuned into a certain direction just by tweaking the surface energies of the underlayer. The simplicity of the methodology in this study will be a powerful knob to understand and optimize underlayers for EUV photoresist-underlayer interactions.
In EUV lithography, spin-on silicon hardmasks have been widely used not only as etch transfer layers, but also as assist layers to enhance the lithographic performance of resist. In this study, we demonstrate a novel approach to functionalize spin-on silicon hardmasks by hybridizing them with functional groups through a sol-gel approach. By varying the concentration and type of the functional groups, the structure and property of the hardmasks can be tuned effectively, especially in the aspects of surface energy, elemental composition, and hardness. The lithographic performance of the functionalized hardmasks was evaluated using NXE3300 EUV exposure system to print line-space features with a targeted CD = 16 nm half pitch. Evidenced by the results, when the functionalized hardmasks were used as underlayers, the resist exhibits large processing window with the printable CD ranging from 11.9 to 19.7 nm and a biased 3-sigma line width roughness = 3.73 nm. In contrast, on a non-functionalized spin-on hardmask, no feature can be printed. Finally, the CF4 and O2 plasma etch rates of the hardmasks were tested to evaluate the impact of functionalization on their etch-selectivity.
A variety of advanced DSA materials have been developed to offer the ease of fabrication and precise control of DSA processes, especially to minimize the number of processing steps and material depositions needed in the DSA implementation. Exemplary materials include spin-on hardmask neutral layer (HM-NL) materials, crosslinkable guiding polymers materials (X-mat), and block copolymers (BCPs) with respective unique combinations of multiple functionalities, e.g., HM-NL materials having etch resistance capability, reflectance control, and non-preferential surface energy to BCP blocks; BCPs with solubility in orthogonal solvents to photoresist, low thermal annealing temperatures required, and no need for top-coat or solvent annealing to get perpendicular orientation. Empowered by these materials, both conventional graphoepitaxy and chemoepitaxy DSA processes can be simplified and enhanced with more potential. For instance, simplified graphoepitaxy DSA by using positive-tone development (PTD) photoresist patterns has been successfully demonstrated. This simplified graphoepitaxy DSA has also led to a facile approach to generate chemoepitaxy guiding patterns.
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