Design, synthesis and testing of materials for 157 nm photolithography
Chambers, Charles Ray
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The microelectronics industries’ ability to keep pace with Moore’s law (the doubling of the number of transistors per integrated circuit every 18 to 24 months) has been due to advances in the photolithographic process. Shrinking the feature sizes that can be printed has been accomplished by decreasing the exposure wavelength which allows higher resolution and thereby more transistors per area. Currently i-line (365 nm) and deep UV (248 nm) are the most frequently used wavelengths for integrated circuit manufacturing. As device geometries shrink below 100 nm, the lithography for critical layers will be performed at an exposure wavelength of 193 nm. The 193 nm technology is being implemented now. Some “next generation” photolithography will be used to print features down to 45 nm. The technology that has investigated the most for printing the 45 nm node is uses 157 nm as the exposure wavelength. As the move is made from one exposure wavelength to the next new photoresist materials are required that have a low absorbance at the lower wavelength. Finding materials for 157 nm photolithography was particularly challenging due to the inherently high absorbance of most materials at this short wavelength. It was discovered that the addition of fluorine into organic molecules greatly increased their transparency at 157 nm. Fluorine or other transparency enhancing moieties were therefore incorporated into analogues of the 193 nm photoresist polymers. New fluoro polymers were prepared by free radical copolymerizations of electron deficient olefins with fluorinated and nonfluorinated norbornenes. Unfortunately the transparency requirements were not fully achieved with the 193 nm photoresist polymer analogues. Highly transparent fluorinated norbornene polymers were prepared using metal catalysts. One such polymer is poly(2-(3,3,3-trifluoro-2-trifuoromethyl-2- hydroxypropyl)bicyclo[2.2.1]hept-5-ene) (PNBHFA). It was discovered that PNBHFA has unique dissolution inhibition properties which are reminiscent of the novolac resin used in two component, non-chemically amplified photoresist system used at 365 nm. A more transparent fluoropolymer (Asahi RS001 polymer) was later introduced. Transparent, highly functionalized additives that could be blended with PNBHFA or the Asahi polymer and used to print high resolution, high aspect ratio images at 157 nm were designed, synthesized and tested.