Electromagnetic radiation in the vacuum-ultraviolet (VUV) region is needed for imaging of very fine features at the 65 nm and 45 nm nodes. Photolithography using 157-nm radiation, emitted from an F2 excimer laser, is a candidate for next generation lithography. Only chemically amplified resists containing fluorinated hydrocarbons and siloxanes have the required transparency at this wavelength. We have identified hexafluoroisopropanol units as a building block for our 157-nm resist polymers. This paper reports our progress on the most recent research development for this platform. The hexafluoroisopropanol functionality, which has a pKa similar to phenol, has been used to increase the transparency of 157-nm single-layer acrylate-based resists. Our recent effort has been focused on the syntheses of new acrylate monomers with highly transparent building blocks based on trifluoroacetone. The first example, a homopolymer derived from trifluoroacetone bearing a fluorinated hemiacetal unit, has moderate transparency at 157 nm (A = 1.9 μm-1). We have also introduced a new acrylate monomer containing a trimer based on trifluoroacetone, where the 6-hydroxy group in the hemiacetal unit is substituted by a fluorine atom, with an acceptable transparency at 157 nm (A = 2.1 μm-1). Copolymers of the former monomer, derived from trifluoroacetone, and tert-butyl α-fluoroacrylate have also been prepared and showed good 248-nm lithographic performance suggesting suitability for 157-nm lithography. This paper will discuss the transparency, etch resistance and chemical properties of several fluorinated acrylate-based resists, synthesized from groups containing pendent hexafluoroisopropanol units and trimers derived from trifluoroacetone.
Norbornene monomers with fluorinated substituents are often used in copolymers targeted for photoresist applications at 157 nm. Homopolymers of these norbornene monomers typically exhibit an absorption coefficient greater than 1.5 μm-1. Comonomers, which are often perfluoroolefins, are needed to meet the transparency requirement for 157 nm lithography, namely an absorption coefficient less than 1.0 μm-1. Clearly, a norbornene monomer that gives a homopolymer with an optical density less than 1.0 μm-1 would require less, if any, perfluoroolefin comonomer, providing a distinct advantage in the production of the base resin. Research in Air Products and Chemicals’ labs has led to the discovery that fluorinated hydroxyalkyl ether derivatives of norbornene ring systems with suitable substitution patterns can give homopolymers with absorption coefficients of less than 1 μm-1. The monomers are produced via a novel reaction pathway involving quadricyclane. This pathway provides a versatile and rich synthetic chemistry, and the potential for eliminating, or at least substantially decreasing, perfluoroolefin incorporation into 157 nm photoresists. Specific examples of these reactions are discussed here, along with VUV-VASE and etch resistance data for a series of polymers derived from quadricyclane reactions.