Mechanisms and Development of Etch Resistance for Highly Aromatic Monomolecular Etch Masks - Towards Molecular Lithography

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Jarvholm, Erik Jonas
Tolbert, Laren M.
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The road map of the semiconductor industry has followed Moores Law over the past few decades. According to Moores Law the number of transistors in an integrated circuit (IC) will double for a minimum component cost every two years. The features made in an IC are produced by photolithography. Industry is now producing devices at the 65 nm node, however, for every deceasing node size, both the materials and processes used are not only difficult but also expensive to develop. Ultimately, the feature size obtainable via photolithography is dependent on the wavelength used in the process. The limitations of photolithography will eventually make Moores Law unsustainable. Therefore, new methodologies of creating features in the semiconductor substrate are desired. Here we present a new way to make patterns in silicon (Si) and silicon dioxide (SiO2), molecular lithography. Individual molecules and polymers, in a monolayer, serves directly as the etch mask; eliminating the photolighographic size limitation of light at a specific wavelength. The Ohnishi- and Ring parameter suggests that cyclic carbon rich molecules have a high resistance towards the plasma process, used to create the features in the substrate. Therefore highly aromatic molecules were investigated as candidates for molecular lithography. A monolayer of poly cyclic hydrocarbons, fullerene containing polymer, and fullerene molecules were created using the versatile photochemistry of benzophenone as the linker between the substrate and the material. First, a chlorosilane benzophenone derivative was attached to the Si/SiO2 surface. A thin film of the desired material is then created on top of the silane benzophenone layer. Irradiation at ~350 nm excites the benzophenone and reacts with neighboring alkyl chains. After covalent attachment the non-bonded molecules are extracted from the surface using a Soxhlet apparatus. Self-assembly, molecular weight, and wetting properties of the material dictates the features shape and size. These features are then serving as an etchmask in a fluorine plasma. The organic etch resist is then removed either in an oxygen plasma or in a piranha solution. AFM analysis revealed that 3 to 4 nm wide defined structures were obtained using C96 as the etch mask. This is about ten times smaller then industry standards. Also a depth profile of 50 nm, which is the minimum feature depth used in industry, was created using a fullerene containing polymer as the etch mask. Directionality and control over the shape and sizes of the features are naturally critical for implementing this technology in device fabrication. Therefore, alignment of the materials used has also been examined. Monolayers of highly stable molecules has successfully been used as etch masks. Further research and development could implement molecular lithography in device fabrication. Self-assembly among other forces would dictate which materials could be used successfully as a molecular resist.
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