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noted in his talk at the conference, the semiconductor industry is continually driven by the need to reduce the size of features on silicon microchips. Reducing the feature size allows more devices to be crammed onto a chip, and that allows faster operation. The patterns on today's computer chips were created in photolithographic processes using 248- and 365-nm light. The semiconductor industry is now transitioning into the next generation of chip fabrication, which exploits light at 193 nm to create circuit lines as narrow as 100 nm or less. The generation after that, in which feature sizes will shrink further to 65 nm and below, will require 157-nm light. This light has to penetrate all the way into the photoresist--the polymeric material that is deposited on top of the silicon wafer and is then chemically modified by the light to form circuit patterns. Photoresists developed for earlier generations of chip fabrication absorb too much light at 157 nm to be useful at this wavelength, Feiring explained, so chemists have had to start looking for new materials. A dozen or so labs, including Co. have been focusing on fluoropolymers because these materials promise good transparency at 157 nm. But, as he explained, transparency is not enough: The photoresist must satisfy several additional requirements because it must perform several different functions during the lithographic process. For example, the photoresist formulation must contain a photoacid generator--an additive that absorbs light and releases protons, which are needed to create the pattern. It also may contain a dissolution inhibitor, an adhesion promoter, and various other additives. Co.'s chemist ticked off 10 characteristics that the photoresist's polymer binder must possess. For example, since regions of the silicon wafer must be etched away while the photoresist covers--and thus protects--other areas, the polymer must be etch-resistant. It must interact with light in such a way as to provide high-contrast patterns. It must be stable at the processing temperatures used and soluble in certain solvents required for processing steps. And so on. Often, these properties conflict. One of the big challenges, is achieving high transparency and good etch resistance in the same material. The 365- and 248-nm photoresists have excellent etch resistance owing to their aromatic groups. But at 157 nm, "you cannot tolerate aromatic groups because they're too highly absorbing. So chemists have had to resort to polycyclic molecules like norbornenes to achieve etch resistance, even though these structures aren't as effective as aromatic groups. At Co. researchers have had success in achieving many of the desired characteristics with copolymers of tetrafluoroethylene, hexafluoroisopropanol-substituted norbornenes, and tert-butyl acrylate. The hexafluoroisopropanol groups are excellent hydrogen-bond donors and thus help make the polymer more soluble in aqueous solutions and more miscible with other polymers, in addition to making the polymer more transparent. The acrylate ester group plays an essential role in the chemistry that "develops" the pattern on the silicon wafer. The Co.'s team also has the option of adding a small amount of a fourth monomer to further tune the properties of the copolymer. "We're not disclosinhas demonstrated that, by using a free-radical polymerization process, it can make these fluoropolymers in 5-kg batches. "We've probably made over 100 kg of polymer" so far what that fourth monomer is at this point. Co. has demonstrated that, by using a free-radical polymerization process, it can make these fluoropolymers in 5-kg batches. "We've probably made over 100 kg of polymer" so far. The company has been providing the polymers to major photoresist suppliers for testing and for the development of photoimaging tools that will be used in 157-nm lithography. In performance tests at this wavelength, Co.'s fluoropolymer resists have been used to pattern lines as narrow as 60 nm. "I think we are the closest that anybody has gotten at this stage to a commercial photoresist product," "but we need all of the properties to be improved." |
| UPDATE | 03.03 |
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