Alternative Method Squishes Out Sub-100 nm Lines
Aaron Hand, Associate Editor -- Semiconductor International, 9/1/2000
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"Exposure tool prices are getting out of hand," said C. Grant Willson, a professor of chemical engineering at the University of Texas in Austin, speaking at the lithography breakfast at SEMICON West in July. Showing the standard progression of tool costs, he pointed out that 193 nm systems were off the line. Next-generation lithography techniques — such as extreme ultraviolet (EUV), X-ray and e-beam — are all particularly expensive procedures as well.
The answer to the cost problem is not going to come from one of these already-proposed tools, Willson contends, but from something less conventional. His alternative is a technique he lovingly calls step-and-squish lithography, although it's officially been dubbed step-and-flash imprint lithography (SFIL). With it, he and other researchers at the Texas Materials Institute and Agilent Technologies Laboratories (Palo Alto, Calif.) have reliably printed features smaller than 60 nm with 6:1 aspect ratios. And they have done so with a system that avoids expensive projection optics.
Willson and his team have designed their system around the shell of an Ultratech 1700 stepper. Their multiple-imprint tool makes more than 30 imprints on a 200 mm wafer with a 1 x 1 in. template.
In SFIL, the substrate — coated with an organic planarization layer — is lined up in close proximity to a standard quartz mask with a low-aspect-ratio patterned chromium surface. The system then deposits a UV-sensitive organosilicon solution between the template and the substrate, and brings the template down lightly on top of the substrate. A broadband UV light source (a mercury vapor lamp with a peak output near 365 nm) illuminates the template through the back, crosslinking the solution. When the template is pulled away, an organosilicon pattern is left from the template relief, which is then etched with a short halogen break-through etch followed by an oxygen reactive ion etch (RIE).
Although similar imprint techniques have achieved fine linewidths in PMMA (polymethyl methacrylate), SFIL overcomes the difficulties those methods have had imprinting large depressed features or thick resist films with high aspect ratios. The RIE is one key to this success because of its selectivity and anisotropy. The new method requires an imprint pressure of only 2 psi and can operate at room temperature.
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One problem with lithography methods that require contact with the wafer is the large potential for contamination. Willson assures, however, that this is not the case with the SFIL technique, which appears to be a self-cleaning process. In one example, several defects (ranging in size from a few microns to more than 180 µm) that appeared on the first imprint had disappeared completely by the eighth imprint (Figure).
SFIL shows potential for use in making a variety of optical devices, such as gratings and polarizers. In fact, the researchers have used the lithography method to fabricate arrays of 100 nm titanium lines with a 200 nm pitch that perform as efficient micropolarizers. •
