New developments in underlayers and their role in advancing EUV lithography

Extreme ultraviolet (EUV) lithography is used to pattern the smallest features in advanced semiconductor devices. The demand for smaller devices with more capabilities requires industry innovation in EUV processes and materials.

By Joyce Lowes, Director, Emerging Materials Technology, Corporate Research and Development, Brewer Scientific.

EUV plays a critical role in the evolution of technology and enables the continuous advancement of the semiconductor roadmap, as it provides the capabilities of higher processing power while using less energy and providing higher performance. However, one of the biggest challenges facing EUV lithography is material requirements, recognizing the critical role underlayers play in the patterning of EUV lithography.

Unlike bottom antireflective coatings (BARCs), reflectivity control is no longer the driving mechanism for underlayers. Now, underlayers are necessary to support resist performance and to enable scaling of the process. Underlayers that offer optimum adhesion while ensuring minimization of pattern defects are key. Beyond just lithography performance, in cases where the underlayers also serve a dual purpose as an etch mask, they need to offer etch resistance beyond their predecessors and at a much reduced thickness.

Brewer Science introduced our first EUV underlayer material in 2010, E2Stack® AL412 material, which serves as the standard for EUV tool qualification and resist screenings. This material delivers excellent adhesion and resist compatibility for lithography. Since then, Brewer Science has introduced a variety of material approaches that offer process and defectivity improvements for traditional EUV lithography schemes. These materials demonstrate improved process window, depth of focus, and LWR/CDU, and can be used across a variety of EUV patterning applications.

Figure 1. Comparison of underlayer properties on traditional CAR and MOR.

Underlayer Challenges for EUV
With the adoption of EUV into more layers and the introduction of high-NA tools quickly approaching, the minimum pitch as defined in DRAM and Logic roadmaps requires photoresist thickness continue scaling down, reaching as thin as 15 nm in the next few years. As expected, the underlying film stack will need to scale as well, all while maintaining existing properties and often doing more.

The industry has already moved from traditional 30 - 80 nm deep UV (248 nm or 193 nm) BARC layers down to 5 - 20 nm EUV underlayers. This thinning-down trend will continue (likely towards 1 – 10 nm) as high NA becomes a reality. Meanwhile, to be able to print ever-shrinking defect-free EUV features with the lowest possible dose while maintaining reasonable process window, more functional demands are being added to the underlayer requirement list. Just to name a few, the underlayer needs to provide enough adhesion to prevent pattern collapse but not too much to cause scum, it needs to be as thin as possible but at the same time provide the required etch selectivity uniformly across a 12-inch wafer, and it is also called on to help reduce the dose but without too much chemical exchange with the resist. Overall, an optimum EUV underlayer design needs to have at least three main challenges in mind: enhancing the lithography performance with compatibility to a wide range of resist vendors and types, thickness and chemical homogeneity control to minimize both coating defects and stochastic impact to the imaging/developing process, and the perfect etch selectivity for pattern transfer demanded by different resist types and integration schemes.