Effects of underlayer on performance of bilayer resists for 248-nm lithography

Abstract

Thin film interference plays an important role in critical dimension control of single layer resists causing large changes in the effective exposure dose due to small changes in optical phase. To overcome these problems bilayer resists have been proposed. Advantages to such systems include enhanced process latitude, enhanced resolution, and improved critical dimension control due to minimization of substrate reflectivity. In this paper, we have investigated the effects of the underlayer with respect to the optical properties as well as the chemical composition on the performance of bilayer resists for 248 nm lithography. The optimum optical constants (index of refraction n((lambda) ) and extinction coefficient k((lambda) )) of the underlayer were deduced by simulations. It was also found that with some underlayers, the optical properties could be tuned by controlling the processing conditions. Novolaks have been found to interact with the resist resulting in significant residue limiting the resolution of the 248 nm bilayer resist to 150 nm. Properly designing the underlayer with suitable optical constants and preventing resist/underlayer interaction resulted in 125 nm resolution with a 248 nm bilayer resist. We also investigated the use of an amorphous diamond-like carbon film as an underlayer material. Thin films, deposited by plasma enhanced chemical vapor deposition, offers advantages over spin on hard baked polymers because it can be deposited conformally with high optical purity. Furthermore, the composition and optical properties can be fine-tuned by changing the process parameters. ©2003 Copyright SPIE - The International Society for Optical Engineering.