Critical process parameters of an acetal-based deep-UV photoresist

Abstract

The present paper gives a first introduction into the resist scheme and the chemistry of a newly developed acetal-based, positive tone deep UV photoresist, called RS 1594/E. The material consists of a phenolic polymer, a dissolution inhibitor, and a photoactive compound. The dissolution inhibition of the polymer is achieved using a poly-N,O-acetal, which undergoes an efficient acid catalyzed bond cleavage. The radiation induced catalytic reaction starts at ambient temperatures and the latent image stabilizes in a dark reaction. The process is completed by the application of a post exposure bake resulting in the formation of highly soluble dissolution promoting fragments, namely aldehyde and alcohol derivatives. The properties of the main resist components have been studied in detail and their influence on the lithographic performance is discussed. In addition, new critical process parameters have been identified and their impact on the lithographic performance of RS 1594/E was analyzed. The dissolution rate ratio between exposed and unexposed resist areas, the post exposure bake temperature, and the delay time between exposure and post exposure bake were determined as the most significant parameters and are discussed in more detail. Prolonged intervals are found to be mainly responsible for the increased formation of an insoluble surface inhibition layer (`T-topping') and a significant linewidth decrease. Compared to state-of-the-art materials, RS 1594/E shows a noticeable improvement in delay time behavior. Even after 60 minutes delay between exposure and PEB acceptable results may be achieved without any formation of `T- shaped' profiles, and a nearly constant linewidth. The surface inhibition effect is markedly reduced by the use of certain organic additives and an optimized photoacid generator. SEM pictures of 0.24 micrometers , 0.26 micrometers , and 0.28 micrometers line and space patterns demonstrate the excellent resolution capability of RS 1594/E.