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This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TSM.2015.2504951, IEEE
Transactions on Semiconductor Manufacturing
Abstract—This paper mainly focuses on establishing the
stability conditions of the double exponentially weighted moving
average (d-EWMA) controller with metrology delay when the
process experiences a general disturbance and analyzing its
control performance under some typical types of process
disturbance. The necessary and sufficient conditions for stability
are established by Routh-Hurwitz criteria. It has been shown that
if the process gain mismatch is greater than 4/3, metrology delay
will make the feasible region of two weighting factors in the
d-EWMA controller decrease, and the larger metrology delay is,
the smaller the feasible region is. Moreover, given two weighting
factors, it is found that metrology delay will weaken the
robustness of the d-EWMA controller. The influence of metrology
delay on the asymptotical control performance is evaluated based
on asymptotical mean square error (AMSE). Optimal weighting
factors and AMSE can be solved based on analytical expression of
the AMSE, which is derived by using the Yule-Walker equations
to avoid function summing limit operation. Through numerical
simulations, it demonstrates that metrology delay may increase
the variability of the process output under certain conditions.
Finally, the Chemical Mechanical Polishing (CMP) process in
semiconductor manufacturing is used to illustrate the validity of
theoretical results.
Index Terms—control performance, double EWMA controller,
metrology delay, run to run control, semiconductor
manufacturing
I. INTRODUCTION
URING the past few decades, as an advanced process
control technique, Run-to-Run (R2R) control has drawn
considerable attention from both industry and academia all over
the world [1]-[3]. In R2R control, data generated from previous
runs are used to adjust the process recipe of the new runs so that
quality characteristics of products can achieve the desired
targets. Until now R2R control has been successfully applied to
Manuscript received May 29, 2015. This work was supported by the
National Natural Science Foundation of China under grant 61203178,
61290323, 61304214 and Samsung Electronics Company.
The first three authors are with the Key Laboratory of System Control and
Information Processing, Department of Automation, Ministry of Education,
Shanghai Jiao Tong University, Shanghai 200240, China (e-mail:
gqs1989@sjtu.edu.cn; gkyang@sjtu.edu.cn; pan_cc@sjtu.edu.cn). The first
and third authors is also with Shanghai Key Laboratory of Navigation and
Location based Services, Shanghai, 200240.
Moon-Sang Lee is with System Engineering Team, Samsung Electronics,
Korea, (e-mail: pantom@kaist.ac.kr).
many semiconductor manufacturing processes, such as CMP
process ([4], [5]), chemical vapor deposition process ([6], [15],
[21]), polysilicon gate etching ([7]).
Due to its simplicity and effectiveness, EWMA-based
controller has already become the most popular R2R controller
in semiconductor manufacturing. Ingolfsson and Sachs [8] first
proposed the basic control structure, which is known as single
EWMA (s-EWMA) controller. They analyzed the stability and
sensibility of the closed-loop process under different kinds of
process disturbance, and showed that the s-EWMA controller
was effective to eliminate shifts in the process. However, in the
presence of process drifts, there would be an offset existing in
the process output when the s-EWMA controller was applied.
Later Butler and Stefani [7] proposed the d-EWMA controller
to deal with this situation. Del Castillo [9] and Tseng et al. [10]
separately investigated the control performance of the
d-EWMA controller, including establishing the stability
conditions and selecting the optimal weighting factors. All the
above work was restricted to the single input single output
processes. In practice, many processes in semiconductor
manufacturing are multiple input multiple output (MIMO) by
nature. Tseng et al. [11] studied the control performance of
multivariate s-EWMA controller in terms of stability and
optimal weighting factors for a MIMO process with no drift.
For different kinds of drifting MIMO process, the stability
conditions of multivariate d-EWMA controller were derived by
Del Castillo and Rajagopal [12], Rajagopal and Del Castillo
[13], and Lee et al. [14], respectively. In addition, Chen and
Wang [15], Fan and Chang [16], Wu and Maa [17], and Pan et
al. [18] also made great endeavors to improve the control
performance of multivariate d-EWMA controller.
The metrology delay is a common phenomenon occurrence
in the feedback control loops. In R2R control environments, it
is very difficult to measure quality characteristics of product
in-situ by current tools. Usually each completed run of wafers is
taken to a metrology station for ex-situ measurement. Since this
post-process measurement strategy is relatively time
consuming, it leads to that measurement information of current
run cannot be used in time to adjust the process recipe for next
run, so metrology delay is inevitable in the feedback control
loops. Metrology delay leads to closed-loop process instability
and degrades the performance of controller. Therefore, it is
necessary to investigate the influence of metrology delay on the
control performance. Several researchers have discussed the
EWMA-based controller with metrology delay. For a MIMO
process with no drift, Good and Qin [19] investigated the
Stability and Control Performance Analysis of
Double EWMA Controller with Metrology
Delay
Qing-Song Gong, Gen-Ke Yang, Chang-Chun Pan and Moon-Sang Lee