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New non-volatile memory, like RRAM, needs high aspect ratio (AR) bi-layer resist (BLR) pillar pattern to act as etch mask for sub-50 nm cell metal pillar definition [1]. HSQ/AR3 resist pillar is still not enough for patterning RRAM pillar since (1) AR is limited at ~7 which is still marginal in etching resistance, (2) BLR resist pillar CD is limited by dry development positive CD-bias, (3) BLR resist pillar is easy to collapse during venting to air, and (4) BLR resist pillar AR is lower for looser pillar densities which degrades the etching resistance. Tone reverse process flow for forming RRAM pillar is developed in this study to overcome these drawbacks. Thermal reflow assists the formation of sub-30 nm contact holes (C/Hs) on ZEP520A e-beam resist. Experimental results are summarized below. All the resolved CDs of C/H on ZEP520A with designed CD (DCD) of 40-100 nm are larger than 100 nm without proximity effect correction (PEC) on blank RRAM film stacking substrate. Smallest etched C/H on LTO after thermal reflow process is 14.4 nm. CD-bias of etched C/H on LTO relative to that of resist C/H after thermal reflow is larger for looser C/Hs. Thermal reflow of ZEP520A is C/H size before reflow, density, sidewall profile and reflow time dependent. NiOx hard mask is apparently without deposited into C/H smaller than 20 nm due to PVD deposition limit since the NiOx dot image disappeared after etching of LTO film for tone reverse. Smallest CD of TiN/Ti/HfOx RRAM pillar by tone reverse process flow is 28.8 nm using NiOx hard mask dot CD of 22.9 nm formed on 20.5 nm etched C/H on LTO after reflow. Uniform CD distribution of designed C/Hs result in uniformly distributed CDs after reflow / LTO etching / RRAM pillar formation. In summary, tone reverse process flow for RRAM pillar formation is successfully developed which has potential for patterning RRAM pillar of CD smaller than 20 nm in the help of NiOx hard mask deposition by atomic layer deposition (ALD) into oxide C/H in the future.
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