Fluoropolymers have been shown to be one of the best materials for high transparency of 157 nm wavelength radiation. Both resists and pellicles are being designed from such materials. One of our approaches to improved transparency for 157 nm resists is based upon fluorinated variations of polymethacrylate and polyhydroxystyrene derivatives. Lithographic studies were carried out on experimental resist platforms using 157 nm and 248 nm steppers, and it was shown that, after selective modification, it is possible to use conventional resist backbones, such as acrylic or styrenic, in the design of single-layer resists for 157 nm lithography. It has been demonstrated in our studies that 157 nm absorbance of these materials can be as low as 1.5-2.0μm-1. Another approach to 157 nm resist design is based upon fluorinated backbone variations. Research will be described focusing on several new monomers having fluorine functions such as -F and -CF3 groups near a polymerizable double bond to improve transparency at 157 nm and to raise the resist glass transition temperature compared to their hydrocarbon analogues. Due to the lower electron density of the double bond, these monomers can be copolymerized with electron-rich vinyl monomers. As an extension to this strategy, we are synthesizing novel fluoropolymers having partially fluorinated monocyclic structures with radical cyclo-polymerization. These polymers have the C-F bond on the polymer main chain and also possess acid labile groups as part of a ring structure to eliminate degassing. In order to further enhance the transparency of these systolic polymers at 157 nm, we have eliminated the carbonyl group. The cyclic nature of the polymer will result in a high glass transition temperature.
Hexafluoroisopropyl alcohol-functionalized acrylate monomers and their (co)polymers were prepared as photoresist platforms for 157 nm imaging. In order to balance transparency with other desirable traits such as etch resistance, we developed several copolymer systems. One is using 2-methyl adamantyl methacrylate as a comonomer, and the copolymer system showed better dissolution contrast compared to the copolymer with tetrahydropyranyl methacrylate without sacrificing transparency. To further improve the absorption properties at 157 nm, monomers having (alpha) -trifluoromethyl group were prepared and polymerized in anionic mechanism. The product polymer was unexpectedly transparent at 157 nm (A = 1.6 micrometers -1) in spite that all the monomers contain carbonyl group. The second system is the copolymer with p-t-butoxy-tetrafluorostyrene. p-Hydroxy-tetrafluorostyrene and p-t-butoxy-tetrafluorostyrene were polymerized radically using AIBN in good yield, and the two resulting polymers showed distinctive solubility differences in aqueous base solution. Finally, this paper describes the synthesis of new monomers having fluorine (e.g CF3- group) in the vicinity of the double bond to improve transparency at 157 nm. Due to the lower electron density of the double bond, these monomers can be copolymerized with electron-rich vinyl monomers using radical initiators.
Tetracyclododecene (TCD) was employed as a key alicyclic structure because TCD-based resist system with high carbon content would show better lithographic performance and better etch resistance than norbornene-based ones. Since hydrophilicity is inherently required in the TCD-based system for good developability, several TCDs functionalized with hydroxyl group were synthesized by Diels-Alder reaction. Calculation by the group contribution methods was carried out in order to estimate the hydrophilicity of the TCDs. Several functionalized TCD-alt-maleic anhydride (MAH) alternating polymers were prepared by free radical polymerization and the alkaline solubility of those polymers were determined using DRM. An optimized resist showed good lithographic performances when a conventional illumination (0.63 NA, (sigma) equals 0.65) is used with a binary mask. The resist could print 110 nm and 100 nm lines on SiON with good proximity bias and large focus latitude. The resist also exhibited better etch resistance than norbornene-based resist system under various etch conditions.
The improvement of the transparency of photoacid generators (PAGs) has an researchers' attention for 193-resist application. In this study, sulfonium salt was chosen as a target PAG. Triphenyl sulfonium salt (TPS) showing strong absorption at 193 nm was selected as a standard PAG. Starting from the ordinary compound, designing several 193-specific PAGs was attempted. Molecular orbital (MO) calculation was used for the estimation of transparency of PAGs. The calculated absorption results fit in with observed absorption spectra of synthesized PAGs in poly(methyl methacrylate) matrix. Furthermore, MO calculation illustrated that polycyclic aromatic systems based on (pi) -system extension concept should be preferable in terms of transparency at 193 nm. A new PAG was synthesized based on the (pi) -extended concept. The resist comprising the new PAG was evaluated in order to verify the effect of transparency improvement. The resist provide good lithographic performances, vertical wall angle without deterioration of photospeed, The MO calculations have materialized a 193-specific PAG for further improvement of resist performances.
A new type of polymerization system for phenolic resin is proposed for high performance positive-working i-line photoresists used for sub-quarter micron lithography. A new design of phenolic resin is also proposed. The effects of end group of phenolic resin are mainly discussed from the standpoint of the dissolution characteristics of the photoresist and their lithographic performances; the resolution of 0.20 micrometers (1L/1.5S) on BARC with an annular illumination aperture (NA equals 0.57, 2.3 (sigma) equals 0.70).
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