Double EWMA控制器是採用固定折扣因子 (discount factor) 來監控生產製程，它雖可使期望製程產出達到目標值，但若產出值的初始偏差過大或所選擇的折扣因子太小，將會導致產出值收斂至目標值的速度太過緩慢，這對於多樣少量的半導體產業將會造成生產成本的增加與重製率上升。針對此問題，Tseng, et al. (2006) 提出了Modified VEWMA回饋控制器，來加快製程的收斂速度且簡化了原本較複雜的公式，但是當漂移率估計不準確時，雖可保證期望產出能漸近收斂，但會造成期望產出與目標值有偏差 (off-target) 的現象。因此本論文希望能建構一回饋控制器，使得製程產出收歛速度加快的同時，又能使期望產出收斂至目標值。

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