Investigation of CD Precise Control in Pitch Doubling Flow for Memory Industry

The conventional pitch doubling process aims to achieve smaller pitch size beyond photo patterning limitation cross technology node for both DRAM and NAND industry. With the shrinkage of chip size, the structure's critical dimension (CD) is more significant to enable the circuitry to function properly. Firstly, this research starts with a brief investigation of the traditional pitch doubling process from analytical study on the origins of CD variation. Secondly, we study the conventional CD control mechanism, such as run-to-run feedback tuning, SPC (Statistical Process Control) basic knowledge and point out its limitations with detailed experimental analysis. Lastly, a novel CD control concept of feed-forward tuning instead of feedback tuning is proposed. And this paper also describes a method that is how to build up the mathematical model for optimal CD control as well as its implementation in industry. Such novel tuning mechanism achieved precise CD control with a dramatic improvement in process capability (process capability index Cpk) with 32% CD variation reduction and 1.4% Yield gain.