Resist materials for 157-nm microlithography: an update

Abstract

Fluorocarbon polymers and siloxane-based polymers have been identified as promising resist candidates for 157 nm material design because of their relatively high transparency at this wavelength. 1 This paper reports our recent progress toward developing 157 nm resist materials based on the first of these two polymer systems. In addition to the 2hydroxyhexafluoropropyl group, α-trifluoromethyl carboxylic acids have been identified as surprisingly transparent acidic functional groups. Polymers based on these groups have been prepared and preliminary imaging studies at 157 nm are described. 2-(Trifluoromethyl)bicyclo[2,2,1]heptane-2-carboxylic acid methyl ester derived from methyl 2-(trifluoromethyl)acrylate was also prepared and gas-phase VUV measurements showed substantially improved transparency over norbornane. This appears to be a general characteristic of norbornane-bearing geminal electron-withdrawing substituents on the 2 carbon bridge. Unfortunately, neither the Ni II nor Pd II catalysts polymerize these transparent norbornene monomers by vinyl addition. However, several new approaches to incorporating these transparent monomers into functional polymers have been investigated. The first involved the synthesis of tricyclononene (TCN) monomers that move the bulky electron-withdrawing groups further away from the site of addition. The hydrogenated geminally substituted TCN monomer still has far better transparency at 157 nm than norbornane. The second approach involved copolymerizing the norbornene monomers with carbon monoxide. The third approach involved free-radical polymerization of norbornene monomers with tetrafluoroethylene and/or other electron-deficient comonomers. All these approaches provided new materials with encouraging absorbance at 157 nm. The lithographic performance of some of these polymers is discussed.

Metal-Catalyzed Vinyl Addition Polymers for 157 nm Resist Applications. 2. Fluorinated Norbornenes: Synthesis, Polymerization, and Initial Imaging Results

“…We have utilized both Ni(II) − and Pd(II) , catalysts in our polymerizations and were able to make several fluorinated, functional polymers for 157 nm lithography applications. , What follows is a discussion of the synthesis and evaluation of three copolymers based on NBHFA, as shown in Figure . Each system has its own weaknesses and strengths, and these will be discussed in turn.…”

“…From studies of other 157 nm polymer platforms, , we discovered that the low molecular weight ketal carbon monoxide copolymers ( 11 and 13 , Figure ) derived from two fluorinated norbornenes are effective dissolution inhibitors. The absorbance of one of these polymers, 11 , as shown in Figure , is 3.6 μm -1 at 157 nm.…”

“…One such approach involves the use of tricyclononene (TCN) analogues. Other approaches to incorporation will be covered elsewhere. ,

13 Scanning electron micrographs of images in resist formulated from NBHFA/TCNCF 3 TBE copolymer ( 24 ).

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“…In addition, judicious introduction of fluorine onto norbornane can cause a very significant reduction in absorbance at 157 nm. Substitution of an electron-withdrawing group such as a fluorine or a trifluoromethyl group at the 2-position of norbornane enhances the transparency more efficiently than substitution at the 7-position (single carbon bridge) …”

Evaluation of fluorinated dissolution inhibitors for 157-nm lithography

“…Because traditionally used photoresist polymers are too highly absorbing at 157 nm, new photoresist chemistry has been explored for 157 nm lithography. 1,2,3,4 Fluorinated materials have emerged as the strongest candidates due to their high transparency at this wavelength.…”