PART II: Litho, metrology, mask/OPC tools make a splash at SEMICON West

by M. David Levenson, Senior Editor

Amid the buzz at this year’s SEMICON West was the launch of what are being billed as “last generation” water-based immersion exposure tools, for 45nm half pitch production. The ASML Twinscan XT:1900i uses a 1.35 NA extension of the in-line catadioptric projection lens of the 1700i, as well as the mature Twinscan platform. Looking ahead to the demands of double-patterning, Martin van den Brink, EVP of marketing and technology, claimed that ASML has always treated CD and overlay metrics with equal criticality, and that ASML’s air-curtain showerhead nozzle facilitated faster scans (600mm/sec), and the Twinscan platform larger throughput, than designs which put the wafers on only one stage. In a clever promotion, ASML also gave away bottles of “ASML hydrolith natural immersion water,” which “contains no bubbles” and is “best before EUV.”

Meanwhile, Nikon claims its competing immersion lithography tool, the NSR-S610C, achieves better depth-of-focus at 45nm because of its slightly smaller (1.30) numerical aperture and Polano illuminator. Hamid Khorram of Nikon Precision reported a single tool overlay specification of <6.5nm and 4.5 m&#955 aberrations in the three-mirror catadioptric projection lens, and said the company has received orders for 20 tools to be shipped in 2007. Gene Fuller of Nikon explored the litho potential beyond 45nm, concluding that increased NA (or a return to 157nm exposure) would be unfeasible given the time available, and that process tricks (such as double exposure) will have to do until EUV or something better comes along.

In a corporate reshuffle, the semiconductor manufacturing parts of Leica have been transformed into a new company: Vistec, with corporate offices in Fremont, CA, and major production facilities in Jena and Wetzlar, Germany and Cambridge, UK. Gerhard Ruppik, GM of the semiconductor systems division, reported that the business is more comfortable in the new management environment than it was as an “orphan” in a conservative German life-sciences and materials company. The Cambridge operation produces high resolution Gaussian beam electron lithography systems for the R&D market, while Jena focuses on higher throughput shaped-beam systems, used today for writing photomasks, nano-imprint templates, and some wafers. In high-resolution resists, structures down to 16nm in width (and 32nm half pitch) can be written using the new SB3050, according to Sebastian Toelg, GM of the electron beam division. The Wetzlar site develops process control products for wafers and masks, including mask metrology, macrodefect, and backside and wafer-edge inspection.

Innovations in lithography were also being touted by small, newer companies at SEMICON West’s Technology Innovation Showcase booths and in private meetings. One such innovation was the mask substrate tuning technology of UCLT, soon to be renamed “Pixer Technologies.” When mask processing or stepper exposure variations create across-chip linewidth variations, the ULCT CDC-101 can correct the problem by changing the transmission of the mask substrate using femtosecond laser technology. The laser writes an array of scattering spots in the fused silica substrate, behind the chrome, with a variable density that alters the transmission by up to 5%, correcting for systematic CD or illumination errors. While such correction takes 5 or 10 hours/mask, it can be done in a wafer fab without removing the pellicle. Two of these tools are now at work at Samsung, according to Gidi Gottleib, UCLT’s director of marketing.

Elsewhere, Luminescent Technologies presented their “inversion lithography” system, which automates optical proximity correction, including placement of scattering bars. Petersen Advanced Lithography showed its EMF3 and Lithopower simulation software that deals with photomask topography and resist complexity over large areas using distributed computing technology. Pixeligent described a reversible contrast-enhancing layer (Arsel) that facilitates multiple exposure lithography without multiple resist processing by bleaching during a laser pulse only where the image intensity is high, thus letting through light to expose the resist, but becoming opaque again before the next pulse. Thus, two patterns can be overlaid in the same resist film — even at the limit where the aerial image is purely sinusoidal, the contrast-enhanced resist exposure is not.

Deep Photonics, a Corvallis, OR-based start-up, described a new class of solid-state DUV laser sources that employ proprietary nonlinear crystals and fiber laser input. While such lasers would not replace excimer lasers in lithography, they might be useful in metrology, cutting and other purposes. The first Deep Photonics system produces 6W of average power at 266nm in a 10 kHz pulsed mode and 0.5W CW, according to James Field, VP of marketing. And Jim Jacobs of Actinix Corp. in Scotts Valley, CA, described a 193nm source for metrology based on more conventional laser technology.

Making news on the metrology front at SEMICON West was ALIS Corp., which decloaked its LG-2 scanning helium ion microscope, promising a revolution in nanometer-scale imaging. According John Notte, senior scientist at ALIS, the microscope employs field ionization of helium gas to produce a stream of particles that can be focused and deflected to scan a target, such as electrons, but with much finer resolution. The field ionization phenomenon is 50 years old, but its use in microscopy has been hindered by the difficulty in stabilizing an electrode with an atomic-scale tip to do the ionization. ALIS (Atomic Level Imaging Systems) has found a way to form a three-atom plateau at the tip of a needle and to use only the ions formed by one of the three atoms as a beam for microscopy with 0.25nm resolution. At the substrate, the 40keV He+ ions produce secondary electrons at the surface, much like an SEM, and collecting those electrons reveals the surface topography. However, since the ions continue deeper, electrons appear only from the area directly irradiated, improving resolution and simplifying analysis of the image profile, for example to find the edge of a step. Rutherford backscattered helium ions can also be collected and the images produced show contrast due to the different scattering cross sections of different materials. Digital photography software can analyze the distribution of brightness in an image to identify the elements responsible. M.D.L.


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