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EUV Leads the Next Generation Litho Race

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As previously reported by Solid State Technology, the eBeam Initiative recently reported the results of its lithography perceptions and mask-makers’ surveys. After the survey results were presented at the 2017 Photomask Technology Symposium, Aki Fujimura, CEO of D2S, the managing company sponsor of the eBeam Initiative, spoke with Solid State Technology about the survey results and current challenges in advanced lithography.

The Figure shows the consensus opinions of 75 luminaries from 40 companies who provided inputs to the perceptions survey regarding which Next-Generation Lithography (NGL) technologies will be used in volume manufacturing over the next few years. “We don’t want to interpret these data too much, but at the same time the information should be representative because people will be making business decisions based on this,” said Fujimura.

Figure 1

Confidence in Extreme Ultra-Violet (EUV) lithography is now strong, with 79 percent of respondents predicting it will be used in HVM by the end of 2021, a huge increase from 33 percent just three years ago. Another indication of aggregate confidence in EUVL technology readiness is that only 7 percent of respondents thought that “actinic mask inspection” would never be used in manufacturing, significantly reduced from 22 percent just last year.

“Asking luminaries is very meaningful, and obviously the answers are highly correlated with where the industry will be spending on technologies,” explained Fujimura. “The predictability of these sorts of things is very high. In particular in an industry with confidentiality issue, what people ‘think’ is going to happen typically reflects what they know but cannot say.”

Fujimura sees EUVL technology receiving most of the investment for next-generation lithography (NGL), “Because EUV is a universal technology. Whether you’re a memory or logic maker it’s useful for all applications. Whereas nano-imprint is only useful for defect-resistant designs like memory.”

Vivek Bakshi’s recent blog post details the current status of EUVL technology evolution. With practical limits on the source-power, many organization are looking at ways to increase the sensitivity of photoresist so as to increases the throughput of EUVL processes. Unfortunately, the physics and chemistry of photoresists means that there are inherent trade-offs between the best Resolution and Line-width-roughness (LWR) and Sensitivity, termed the “RLS triangle”.

The Critical Gases and Materials Group (CGMG) of SEMI held a recent webinar in which Greg MacIntyre, Imec’s director of patterning, discussed the inherent tradeoffs within the RLS triangle when attempting to create the smallest possible features with a single lithographic exposure. Since the resist sensitivity directly correlates to the maximum throughput of the lithographic exposure tool, there are various tricks used to improve the resolution and roughness at a given sensitivity:  optimized underlayer reflections for exposures, smoothing materials for post-develop, and hard-masks for etch integration.

Mask-Making Metrics

The business dynamics of making photomasks provides leading indicators of the IC fab industry’s technology directions. A lot of work has been devoted to keeping mask write times consistent compared with last year, while the average complexity of masks continues to increase with Reticle Enhancement Technologies (RET) to extend the resolution of optical lithography. Even with write times equal, the average mask turn-around time (TAT) is significantly greater for more critical layers, approaching 12 days for 7nm- to 10nm-node masks.

“A lot of the increase in mask TAT is coming from the data-preparation time,” explained Fujimura. “This is important for the economics and the logistics of mask shops.” The weighted average of mask data preparation time reported in the survey is significantly greater for finer masks, exceeding 21 hours for 7nm- to 10nm-nodes. Data per mask continues to increase; the most dense mask now averages 0.94 TB, and the most dense mask single mask takes 2.2 TB.

—E.K.



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