Metal oxide resists (MORs) have become one of the most attractive photoresist platforms that allow for high resolution and etch bias of small features while having a robust lithography performance. In this work, we present our study about improving line fidelity and reducing the dose of MOR for line space EUV lithography by applying spin-on underlayers (ULs). It is known that MOR patterning is induced by the activation during exposure and condensation of the active sites. Herein, we discuss the influence of ULs on MOR performance. A series of ULs with various chemistry, thickness, or process conditions were screened with MOR using NXE3400 EUV exposure system to print 14-nm HP line-space features. The results show that the nanobridges and scum can be alleviated, while the remaining resist thickness after development can be improved by up to 20% along with 5 to 10% dose reductions. A comprehensive assessment of the ULs encompassing various chemistries examines coating quality, uniformity, and surface energy. The discussion delves into the correlation between the surface properties including morphology, interaction, etc., and their respective impacts on lithography performance. Lastly, some spin-on ULs produce up to 75% reduction of metal diffusion from the MOR into the underlying layers.
Spin-on glass (SOG) underlayers to enhance extreme ultraviolet (EUV) lithography for patterning below 28-nm pitch require an in-depth understanding of the required adhesion forces necessary for good lithography. Here, we proposed a fundamentally new SOG underlayer platform composed of polymer blends that can achieve superior uniformity to improve line fidelity and provide a design path for underlayer materials. The structure and property of elemental composition and surface energy can be controlled easily and precisely by varying the combination of polymer, and consequently understanding and tuning the lithographic performance. The lithographic performance of SOG blends was evaluated using NXE3400 EUV exposure system to print 13-nm and 14-nm HP line-space features and contact hole features with CD of 20-nm. The results show that the polymer blends expand the process window for EUV resist for line-space with printable CD >11 nm and biased LWR to 3.6-nm without impact on dose when compared to conventional copolymers. In the case of CH patterning, CDU and DOF improvements were observed when compared to conventional SOG copolymers. Systematic studies on polymer blends based on functional groups and formulation compositions are in progress to establish a better understanding of enhancement of EUV lithography.
In EUV lithography, good resist patterning requires an assist layer beneath it to provide adhesion to prevent pattern collapse of small features and allow for higher aspect ratios. In addition, future EUV high numerical aperture (NA) is expected to require a decrease in thickness from the overall patterning stack. In this study, we are exploring a fundamentally new approach to developing an alternative and cost-effective underlayer to functionalize surfaces and enable EUV patterning. Rather than forming a 5-nm polymer film between the resist and its substrate, we propose to modify the substrate by spin-coating a thinner layer. In contrast to conventional underlayers (5–10 nm), the substrate is modified by a sub-1-nm layer during baking. Comprehensive analysis of the surface modification and coating was conducted by GPC, ellipsometer, and contact angle to identify the structure, stability, coating quality, and surface energy. Lithographic performance of existing EUV resist with the assist of this thin layer on Si wafers and different silicon hardmasks was evaluated using NXE3400 EUV exposure system to print HP14nm line-space features. It has been demonstrated that this sub-1-nm layer is able to realize HP14nm with a wider process window, higher depth of focus, and lower LWR on a Si wafer. Moreover, a silicon hardmask that could not realize printable features had significantly improved lithographic performance with the assist of this layer. Comparisons were also made with the industry-standard HMDS priming. Systematic analysis indicates that a sub-1-nm layer exemplifies a novel and effective way to enhance photoresist-substrate compatibility and improve EUV lithographic performance.
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.
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