MRS Spring Day 2: Combinatorial screening for Ta metal gates, supercritical CO2 tricks, CMP’s too thirsty

by Michael A. Fury, Techcet Group

April 15, 2009 – Day Two of the MRS Spring meeting welcomed papers and discussion spanning nanocrystal structures for memory devices, combinatorial screening for metal gate alloys contacting a HfO2 gate dielectric, more uses for supercritical CO2, and various aspects of chemical mechanical planarization, including work with exotic niobium wafers for RF applications and CMP’s thirsty environmental impact.

Thierry Baron and colleagues from CNRS, CEA-Leti, and Atmel demonstrated the viability of nanocrystal structures for advanced flash memory devices. These incorporate heterogeneous domains into the gate dielectric in order to increase charge trapping and provide a 50% greater memory window. A variety of nanocrystal materials have been evaluated, including Si, Pt, Ni, Ge and InAs. Particularly elegant was the formation of a regular hexagonal array of Pt nanodots, 22nm diameter on a 40nm pitch, using a PMMA-PS phase separation copolymer as a template.

K-S Chang, of NIST, U-MD, Brookhaven, and IMEC have developed a novel combinatorial screening method for rapid characterization of a continuous range of Ta metal gate alloys that can be in direct contact with a HfO2 gate dielectric. The alloy grade consists of sputtering a wedge layer with a retracting shutter using a Ta target, rotating the substrate 180°, then repeating with a C target, all in the presence of N2 for reactive sputtering. However, each pass is only allowed to deposit to a thickness of one crystal lattice. The total gate thickness is built up by repeating this Ta/C operation pair as often as necessary; this ensures a homogeneous alloy composition vertically through the gate at each location. Test devices are then fabricated and characterized by isolating individual gates and mapping the device performance against the composition map.

Christopher Ober and a team from Cornell presented their work on bio-inspired polymer “brushes” designed to selectively protect surfaces against cell adsorption. These consist of nucleation points on a gold surface that anchor long-chain PEG-modified styrene. Modified with functional groups, these structures can serve as sensors for specific molecules in solution.

Chung Seung-Min of Japan’s Tohoku University took a critical look at the copper diffusion barrier that forms in situ from a Cu-Mn alloy deposited directly on SiO2. He found that the dielectric constant of the MnO barrier itself was quite high, ranging under various process conditions from 5.2 to 11.5 vs. the 4.2 of the SiO2 dielectric, yet the overall dielectric constant of the combined system decreased by 5%. An explanation of this apparent contradiction was deferred to ongoing experimentation.

A number of papers were presented on various aspects of CMP, including a commanding show of five papers co-authored by Araca Inc, a startup formed in 2004 in Tuscon, AZ with ties to Japan’s Toho Engineering. Topics ranged from chip fab concerns such as pad wear, slurry dwell time in grooves, and characterizing actual pad-wafer contact area, to the use of CMP for non-semiconductor materials and applications. Perhaps the most exotic of these was the work by Sinan Muftu of Northeastern University on niobium wafers. This work is critical to improving the performance of superconducting RF cavities in linear accelerators, where surface asperities and microcracks diminish the current-carrying capacity of these large devices. The reward for their efforts is an RMS roughness of 39nm, a record for niobium surfaces.

Supercritical CO2 is still finding uses in material development, in this case by A.H. Romang and J.J. Watson at U Mass Amherst. Using a mixture of two distinct block copolymers as a template, they have fabricated a low-k dielectric film with k values as low as 2.2 that can stand up to the rigors of a CMP process. Implementation outside of a laboratory environment remains to be addressed.

The CMP semiconductor applications world was put into perspective by Dave Dornfeld of UC-Berkeley, who spoke on the sustainability of CMP in an increasingly environmentally aware industry. CMP operations account for only 0.1% of fab electricity usage, and 0.2% of the waste heat generated — but 17.7% of the water consumed (though still a distant second to wafer cleaning at 72.3%). He advises that the CMP community would be wise to take action to reduce this consumption on its own terms, rather than have others impose demands from the outside. In Copernican fashion, he posed the question: Is the environment part of the economy? Or is the economy part of the environment? This perspective can be viewed as similar to the early debates about the sun revolving around the earth.

Perhaps the most striking observation of the day was in the audience. An attendee rushed late into a presentation pulling in one hand his oversized briefcase on wheels, and in the other hand carrying an open laptop: screen glowing, power cords dangling, and a cup of coffee balanced on the keyboard. Now that is real confidence!

Michael A. Fury, Ph.D, is senior technology analyst at Techcet Group, LLC, P.O. Box 29, Del Mar, CA 92014; email [email protected].


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