157 nm Lithography

The absorbance of a fluorine containing monomer at 157 nm is influenced by the position of the F atom in the molecule. For norbomene, for example, one of the commonly encountered monomers in polymers designed for 157 nm lithography, the absorbance varies greatly depending on the position of the substituting F atom. When two F atoms are introduced in a molecule, the absorption is reduced even more [62]. 

As described in detail later, the polymers for use in 157 nm lithography universally contain fluorine because only fluoropolymers provide low enough absorption at 157 nm. The acid group of choice is hexafluoroisopropanol, which has a pKa similar to that of phenol, as mentioned earlier. Several platforms are available (Fig. 41) 1) tetrafluoroethylene-norbornene,2) 2-trifluoromethyl-acrylate-norbornene, 3) 2-trifluromethylacrylate-styrene, 4) 2-trifluoromethy-lacrylate-vinyl ether, 5) all-norbornene, 6) methacrylate, and 7) cyclopolymers. 

For further enhancement of the resolution at 193 nm, immersion lithography employing water as an optical element between the last lens and resist has emerged recently as a new technology in competition with 157 nm lithography. In immersion lithography the effective exposure wavelength is reduced by the refractive index of the immersion fluid (193/1.44= 134 nm for 193 nm water immersion). 

Fluoropolymers for 157 nm lithography can be categorized into two groups 1) polymers containing F in the backbone, typically prepared by copolymerization involving tetrafluoroethylene (TFE) and 2) polymers containing F in the side chain (Fig. 83). NBHFA has been copolymerized with NBTBE,NBHFA... 

Completely new polymer architecture for 157 nm lithography has been proposed on the basis of quantum chemical calculation of VUV absorption of a number of model compounds [315]. Time-dependent density functional theory (TD-DFT) calculations suggested that sulfonic acid esters are transparent in the 157 nm region. In fact, poly(vinylsulfonyl fluoride) and poly(methyl vinyl-sulfonate) have been found to show low OD157 of 2.1 and 2.2/pm [316]. Various... 

Fig. 178 Scanning electron micrographs of sub-100 nm features printed in chemical amplification resists 60 nm line/space patterns by phase shifting 157 nm lithography [511], 70 nm line/space patterns by X-ray lithography [513], and 70 nm line/space patterns by EUV lithography [514]...

SEMATECH (2003) 4th International Symposium on 157 nm Lithography, August 25-28,2003, Yokohama, Japan CD available from Selete/International SEMATECH... 

Poly(methyl methacrylate)-hased resists comprise a significant fraction of positive resists used in 193-nm and 157-nm lithographies. These resists are largely based on the acid-catalyzed deprotection of tertiary ester groups. 

This polymer generates hexafluoroalcohol on aeid-eatalyzed thermolysis and tends to have low eontrast. Addition of tert-hutyl ester oligomer into the formulation as a dissolution inhibitor helps to improve the development eontrast of the resist this partieular version, marketed by Clariant Corporaton, was the first commereially available resist designed for 157-nm lithography... 

Chart 9.6 Chemical structures of random copolymers used for 157 nm lithography (a) poly[4-(2-hydroxy hexafluoro isopropyl) styrene-co-t-butyl acrylate] and (b) poly[4-(2-hydroxy hexafluoro isopropyl) styrene-co-t-butyl methacrylate] [35]. 

TABLE 57.2. Optical constants and other properties of polymers for 157 nm lithography. 

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