Next-Gen Metrology: Searching For A Bright X-Ray Source

By Debra Vogler

Metrology for semiconductor applications is a broad topic regardless of whether one is talking about front-end-of-line (FEOL) or back-end-of-line (BEOL) technologies. Benjamin Bunday, project manager, CD Metrology and senior member of the technical staff at SEMATECH, broke down the topic of next-generation metrology at 10nm and below into four main categories for SEMI:

• Imaging CD metrology
• Scattering CD/3D metrology
• Films metrology
• Defect inspection/review

For high-volume manufacturing (HVM), these technologies will need to be automated, high-throughput, and non-destructive. In particular, two of these — scattering CD/3D and films — could benefit if the industry could develop a bright, high-energy X-ray source. SEMATECH, along with its partners, has done an extensive gap analysis for CD metrology beyond the 22nm node (see Table 1).

Table 1: Established & Emerging X-ray Metrology Applications

Source: SEMATECH white paper study; Summary table of X-ray metrology capabilities.

Some of the key points for 10nm and below were discussed by Bunday in an interview with SEMI in anticipation of his presentation at SEMICON West 2015 at the Semiconductor Technology Symposium (STS) session on advanced lithography (“Making Sense of the Lithography Landscape: Cost and Productivity Issues Below 14nm and the Paths(s) to 5nm”) on Wednesday, July 15, 10:00am-12:30pm).

When it comes to next-generation scatterometry, one of the challenges the industry needs to address includes the possibility of n and k shifts that arise from quantum confinements and very small targets. Additionally, scatterometry must deal with more complex device architectures. Bunday told SEMI that the industry has identified a solution that gets around these problems — critical dimension small angle X-ray scattering (CD-SAXS). “But we don’t have a bright enough x-ray source to do it quickly enough,” said Bunday, “so throughput is a problem.”
While a synchrotron could provide such a source, it certainly is not feasible in a fab. “There are a few ideas for really bright sources that start becoming more economically feasible, and this is an important problem that deserves attention from the industry.” Addressing next-generation scatterometry is not the only technology that would benefit from such a source. “We double-down on this for the films metrology challenge too — a bright X-ray source will be needed there as well.”

Bunday continued, emphasizing the critical need for X-ray sources, “For decades, X-ray metrology people have been dealing with little incremental improvements to X-ray sources (i.e., maybe 10%).” Bunday told SEMI, “What we need is a 100X or better revolution in X-ray sources — the more the better for HVM at 10nm going to 7nm and 5nm.”

Pointing to the “current speed” column in Table 1, Bunday noted that for the XRD technique, with today’s sources, it takes 30 minutes to a few hours per wafer. “That’s fine for doing a few wafers here and there for process development,” he commented, “But if you want to do process control on every lot, that becomes a problem.” He also explained that a metrologist might have the need for more spatial resolution — perhaps mapping the XRD results and map the strain, which would require more time. “So brighter sources can help achieve some of these things,”

Another benefit that would come if the industry had a brighter X-ray source would be powering vapor phase decomposition-total reflection X-ray fluorescence (VPD-TXRF), which presently takes on the order of an hour. Furthermore, it’s a destructive technique. “If you have a much brighter source, you can possibly bring that measurement in-line and do so nondestructively, and do a lot more of it,” said Bunday.

Reviewing the other X-ray metrology applications listed in Table 1, it’s clear that all of them would benefit from a brighter X-ray source. “If this source were for lithography, it would be an easy situation to get funding,” said Bunday.

He further observed that the industry is reticent about parting with money for metrology. “But if you’re looking to invest towards a high-impact metrology development, spend money on a brighter, high-energy X-ray source, because it will impact all five of the metrology technologies (see Table 1),” Bunday told SEMI. “This will go a long way towards providing the needed HVM advanced films and CD/3D metrology for upcoming nodes.”

SEMICON West 2015 (July 14-16 in San Francisco, Calif.) will feature an STS program focusing on advanced materials and processes on Wednesday, July 15. The morning session will explore photolithography issues at 14nm and below. Transistor scaling will be examined in a session in the afternoon. Learn more at www.semiconwest.org/node/13861. Register now! Discounted registration (save $100) through June 5.