VUV metrology sees ultra-thin films

by Ed Korczynski, senior technical editor, Solid State Technology

Spectroscopic reflectometers (SR) and ellipsometers (SE) provide thickness and composition information for thin dielectric films. As the industry has moved to ever thinner films, there is less material to refract light and SE signals drop off. Metrosol, one of last year’s SEMICON West Technology Innovation Showcase winners, is now ready to make a splash, providing vacuum-ultraviolet (VUV) SR tools that can resolve information from extremely thin films.

The company’s big launch at SEMICON West “is only half of our launch,” said CEO Kevin Fahey, in an interview with WaferNEWS. “Customers are used to new technology being first used in R&D. Even though we’re a new technology, we’re designed for high-volume production and we’re moving down the commercial path with several tier-1 guys.”

The two immediate applications for VUV metrology down to 120nm are high-k dielectrics for both gates and memory-storage. “Once you understand the optical properties, you can expect to see significant differences in even 10-Angstrom films,” claimed Fahey, who was brought on earlier this year from FEI Co.

“New technology always allows for new use cases. If you look at what we’re able to do by working with VUV, in the wavelength range below 190nm all the dielectric materials have their peaks in the absorption curve,” explained Dale Harrison, CTO and co-founder of Metrosol. “Since we can tell where the peaks are, in a single-measurement we can resolve both composition and thickness.”

A typical SE covers the 1-5 eV energy range, while just the 120-190nm range covers 5-10 eV. “If you convert wavelengths into electron volts, you’ll see that we’re doubling the information space,” explained Harrison. [However, some broadband SEs now resolve down to 150nm. — EDIT 7/27/2007]

“In the VUV, you’re working in a range where a change in composition changes both n and k so we can decouple thickness from composition,” explained Fahey. “If you go to short-enough wavelengths, everything is absorbing.”

The tool needs to use reflective optics, and vacuum with different atmospheres, since quartz lens elements are not transparent below 190nm. The overall hardware design is based on the Asyst EFEM modular architecture. Throughput for one chamber may be ~25 wafers/hr, and up to a total of five chambers can be used. Hot-swap capability for chambers is planned by 2Q08 to aid in overall system uptime. The first five beta units of the fully-automated system will be ready this September. Two manually-loaded chambers have been in use for over two years at customers, one for hafnium-silicon-oxide films and one for nanoimprint lithography (NIL) characterization.

Compared to other production metrology systems for hafnium-silicon-oxide high-k gate dielectrics, the purchase cost is claimed to be one-half to two-thirds that of an x-ray or extended range ellipsometer tool, and typical throughputs are 2&#215-10&#215 of such systems. Reliability data from three years of running the basic chamber show good MTBF and reasonable PM schedules. –E.K.


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