by M. David Levenson, Editor-in-Chief, Microlithography World
The location chosen for the fourth International Symposium on Immersion Lithography, sponsored by SEMATECH, IMEC and Selete, illustrated the prominence gained by this manufacturing technology since its founding meeting in Bruges, Belgium — the site in Keystone CO, was 9300 feet above sea-level Bruges, by far the highest altitude for any lithography conference. The 251 attendees heard that three types of 193nm water immersion exposure tools are being used for mass production: Nikon NSR609Bs for 55nm and 610Cs for <50nm NAND at Toshiba, and multiple ASML 1700i's at Samsung for ~50nm chips. Solutions for the few remaining difficulties are at hand, and machines designed to achieve the maximum resolution possible with water as the immersion fluid are being delivered in time for the 45nm chip generation.
However, 45nm may be the high-water mark; the course beyond that seems murky. Exposure wavelength will remain fixed at 193nm, so media with higher refractive indices are needed if dimensions are to shrink in single-exposure lithography. Synthetic (Generation-2) immersion fluids with indices around 1.64 (14% higher than water) have appeared, but development of suitable lens materials is delayed. Worse, the ~10% resolution improvement possible with Gen-2 immersion is insufficient for 32nm production, for which the immersion fluid, lens material, and resist must all have an index of refraction near 2.0. While one of the potential lens materials has such an index, John Hoffnagle of IBM deduced a possibly fundamental reason why suitable Gen-3 fluids and resists cannot be made.
Thus, symposium chairman Bryan Rice, an Intel assignee to SEMATECH, visualizes an insertion point for high-index (Hi-n) immersion at the 22nm node, with double patterning, giving the crystal growers and resist chemists more time for R&D. Roger French of Dupont claimed that Gen-2 technology could improve the production economics even for cases like 22nm double patterning, where water immersion seems theoretically feasible. The question is whether that improvement will be enough, or whether some other development — like EUV — might make high-index immersion irrelevant by the time it is ready for production. A decision point on Gen-2 exposure tool development is coming early next year, according to Diane McCafferty of ASML.
The properties of plausible lens materials must be understood before any such decision is made — and it is particularly disappointing that the absorption of the favorite Hi-n material, LuAG (lutetium aluminum garnet, n=2.15), remains twice the target needed to demonstrate feasibility, and more than 20X that possible for a production system. The problem, according to Lutz Parthier of Schott Glass, is obtaining consistently pure starting material. Still, Schott has now grown an 80mm x 300mm boule of LuAG using a core-free process that keeps the index uniform, and the attenuation (0.11cm-1) is sufficient for laser damage studies and interferometric lithography experiments. Prisms for interferometers and samples for test will be fabricated by the end of the year, Parthier reported. He also set the goal of producing the first 150mm diameter lens blank before the end of 2009.
The alternative lens crystal is BaLiF3, being developed by Tokuyama Corp. in Japan, with a lower index (n=1.64) that makes it suitable for Gen-2, though not Gen-3, systems. It is, however, related to widely used materials such as CaF2. Growing uniform, strain free, low attenuation crystals of BaLiF3 has proved challenging. Toshiro Mabuchi of Tokuyama reported in a poster that 150mm diameter single crystal boules have been grown and annealed to push attenuation below 0.003cm-1 and stress birefringence below 1.7nm/cm.
Laser durability and homogeneity tests are underway, with a final assessment planned for mid-2008.
Organic Hi-n fluids have higher viscosity and lower surface tension than water, restricting the scan speed of current local-fill immersion technology. Alternatives include a wafer-wet design or a scheme where droplets left behind during the exposure scan are mopped up later. Both require that the fluid and resist do not interact over fairly long time periods, reported Diane McCafferty of ASML. Taiichi Furukawa of JSR reported that low-attenuation Gen-2 fluids had been synthesized and specially developed topcoats resulted in contact angles above 70°, potentially speeding throughput. Technologies to purify and de-gas the expensive Gen-2 fluids for re-use had been shown to work, according to several authors.
Gen-3 fluid, however, is another matter. Chemical schemes to increase the index of refraction of liquids had not given high enough indices or low enough attenuations at 193nm. So, the plan had become to suspend nanoparticles of high-index solids (like LuAG) in the immersion fluid. John Hoffnagle pointed out that a fundamental relationship between the index increase, turbidity, and light scattering of such composites had been known for a century. He derived an inequality that showed that the nanoparticle dimensions have to be very small — as small as a single molecule — if 10% of the light is not to be scattered, or the “fluid” congeals into moist nanoparticle sand.
A panel discussion on Hi-n materials agreed that cost and timing were key concerns. Will Conley of Freescale Semiconductor and Harry Sewell of ASML both advocated increased R&D funding to make materials available when they will be needed. One problem is that the organizations capable of such advanced research were terribly burned in the 157nm lithography debacle and are wary of the financial dangers of such speculative large-scale projects. Another is the timing: L.J. Chen of TSMC pointed out that his company would need an exposure tool in 2009 for 22nm process development. A Gen-3 (NA=1.85) prototype would support single exposure technology with k1=0.31 — otherwise, double patterning would be adopted. The state of Hi-n technology reported at this symposium suggests a tremendous flood of innovation will be needed.