Nikon Fellow Discusses Means to Extend Immersion to 10 nm Half Pitch
At the recent LithoVision symposium, Soichi Owa, Nikon Fellow and Technology Development Section Manager, presented his view on Immersion Lithography Extension Down to 10 nm Half Pitch. Owa reported that although source mask optimization (SMO) can be very effective in extending process window (on the order of 15-25%), he cautioned that the effect of SMO on pattern shrinkage is limited to ~15% or approximately a half node shrink improvement. In contrast, Owa highlighted that multi-patterning enables dramatic pattern shrinkage, with double patterning (DP) capable of shrink factors of 0.5X - enabling immersion extension to two additional technology nodes; and quadruple patterning being capable of a 0.25X shrink, thereby enabling immersion extension for potentially four further nodes.
Figure 1. SMO can be very effective in extending process window (left image), but has limited pattern shrinkage benefit.
Although many flavors of double patterning have been proposed, Owa referenced various industry reports of sidewall (spacer) DP being been adopted for ~30 nm NAND flash production, and attributed this to its less stringent overlay requirements and manufacturing cost effectiveness. Owa also referenced work by Hynix Semiconductor (SPIE Advanced Lithography 2009) that proposed a spacer doubling approach for NAND Flash with extension to the 19 nm half pitch (hp) expected; as well as a presentation from Sam Sivakumar of Intel Corporation (Immersion Extensions Symposium 2009) discussing a spacer solution which could extend the 40 nm hp to 20 nm using double patterning, and on to the 10 nm hp using quadruple patterning.
Owa then transitioned to a detailed look at 193 nm immersion solutions for the various technology nodes. When considering line space (L/S) formation for both NAND and logic devices, he reported that single exposure is sufficient for the 45 nm hp, with double patterning then being adopted at the 32 nm hp. Looking next at the 22 nm node for a logic (SRAM gate) pattern, Owa proposed extending immersion capabilities using a line cutting solution with positive tone resist consisting of 2-exposures and 2-etches. Using this method, depth of focus (DOF) simulations predicted extendibility to the 18 nm hp.
Figure 2. Owa proposed extending immersion capabilities using a line cutting solution with positive tone resist consisting of 2-exposures and 2-etches (left image). Using this method, DOF simulations predicted extendibility to the 18 nm hp.
Looking further ahead to the 16 nm node under these same conditions, Owa discussed a line cutting solution consisting of 4 exposures and 4 etches, which simulations predicted would support extendibility to the 14 nm hp. To proceed beyond that to ~10 nm hp, he commented that when using positive tone resist, line cutting using 8 exposures and 8 etches would be necessary.
Considering the processing complexity of these methods using positive tone resists, Owa then highlighted the potential for negative tone resist for line cutting applications. He reported that dark field patterns have an advantage in dose latitude that enables imaging of very narrow lines, and he further explained that by combining negative tone resist with multiple exposures, excellent cutting holes can be achieved. The potential for this negative tone method was demonstrated with simulations predicting applicability down to the 10 nm half pitch. He also highlighted the productivity benefits of this potential method since the etch steps would be halved as a result of switching to the negative tone process.
Figure 3. Owa showed the potential for a negative tone method with simulations predicting applicability down to the 10 nm hp (left image). He also highlighted the clear productivity benefits of this alternative method which cuts the etch steps in half.
Owa concluded his presentation by stressing that 193 nm immersion is indeed extendable to 10 nm half pitch for SRAM patterns, using spacer quadrupling as well as line cutting lithography solutions. He summarized that negative tone resist development and mask costs for multiple exposures are key points of concern though, and noted that industry efforts in those areas are vital to advance this technology.