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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