ASML “double-dips” with updated litho tool

by M. David Levenson, Editor-in-Chief, Microlithography World

Dec. 10, 2008 – Double patterning immersion lithography will be the dominant manufacturing paradigm at , and ASML announced at last week’s SEMICON Japan (Dec. 3) a new exposure tool designed for the most litho-intensive version of it: LELE, or “double-dip.”

The NXT 1950i employs the same 1.35NA (38nm nominal resolution) projection lens as earlier ASML immersion steppers, but incorporates the new NXT twin stage system along with dramatic changes in stage positioning technology and an improved fluid control system. Its 200 wafers/hour maximum throughput means that even when a wafer must be immersed twice at the exposure tool to print a critical layer, volume is expected to remain acceptable.

The two stages of the NXT platform magnetically levitate themselves over a floating permanent magnet plate that functions as the reaction mass, according to Frank van de Mast, ASML senior product manager. Linear actuators built into the levitation system propel the stages across the magnet plate. Short stroke motors above the levitation and propulsion coils and below the wafer chuck provide the required nanometer precision. Since the two stages can accelerate quickly and butt into one another beneath the immersion head while the fluid continues to flow, less time is needed for wafer transfer.

Figure 1: The 0.6nm alignment reproducibility (a) with the grid plate measurement system is uniform across the wafer and does not depend on stage position. Over 24 hours the grid stability is better than 0.15nm. (Source: ASML)
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Stage positioning is done with excellent reproducibility and stability (Figure 1) with a grid plate encoder positioned just above the wafer stages for stability (Figure 2). Four two-degree-of-freedom sensors on each wafer stage together read the 6°-of-freedom that define the wafer position from the encoder. Air turbulence cannot disrupt measurements as with conventional long-path interferometers, resulting in much improved overlay precision. For double-patterned wafers returned to the same chuck, the NXT 1950i boasts a 2.5nm (mean + 3σ) overlay spec, with only 0.5nm (two silicon atoms) greater error for alternate chucks.

Figure 2: Grid plate location above wafer chuck and attached to metroframe and projection lens. (Source: ASML)
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Figure 3 shows overlay results obtained with the prototype NXT:1950i system. In double-dip lithography, overlay errors become CD errors. Thus, a 10% CDU spec for 32nm node chips requires an overlay spec below 3.2nm for LELE and LPLE double-patterning techniques.

The immersion fluid control system has also been improved, yet again, on the 1950i. The immersion head is now diamond-shaped, according to van de Mast, with the meniscus stabilized by direct air pre-loading. Since there is no longer a stopper disk interrupting flow on wafer exchange, defectivity is reduced by a factor of 2×-3× and overhead is also lessened. While the maximum scan speed remains at 600mm/sec, greater acceleration is now possible, improving throughput.

Figure 3: Overlay accuracy of the NXT:1950i exceeded the 2.5nm spec for both one and 3 days. (Source: ASML)
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The NXT:1950i is targeted at >200 wafers exposures per hour, 30% more than today’s most advanced systems. ASML expects the NXT:1950i to print well over 4000 exposures per day in volume manufacturing of memory chips. Of course, when each chip level requires two exposures, the wafer throughput will be half or less of the maximum for single exposure, but still a tolerable 2000 per day.

With high throughput and 38nm L/S resolution, the NXT:1950i is also expected to be cost-effective for leading-edge single patterning and spacer-layer DPT applications. In the future, the NXT stage platform will be adapted for EUV exposure, according to van de Mast.

ASML has received multiple orders from customers for the NXT:1950i and has scheduled first shipments for the first half of 2009. — M.D.L.


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