Christopher A. Bode

Survey Automatic control in microelectronics manufacturing: Practices, challenges, and possibilities

Advances in modeling and control will be required to meet future technical challenges in microelectronics manufacturing. The implementation of closed-loop control on key unit operations has been limited due to a dearth of suitable in situ measurements, variations in process equipment and wafer properties, limited process understanding, non-automated operational practices, and lack of trained personnel. This paper reviews the state-of-the-art for process control in semiconductor processing, and covers the key unit operations of lithography, plasma etching, thin film deposition, rapid thermal processing, and chemical-mechanical planarization. The relationship of process (equipment) models to control strategies is elaborated because recently there has been a considerable level of activity in model development in industry and academia. A proposed control framework for integrating factory control and equipment scheduling, supervisory control, feedback control, statistical process control, and fault detection/diagnosis in microelectronics manufacturing is presented and discussed.

Run-to-run control and performance monitoring of overlay in semiconductor manufacturing

In the manufacture of semiconductor products, overlay is one of the most critical design specifications. Overlay is the position of a pattern relative to underlying layers, and overlay control largely determines the minimum feature size that may be incorporated into semiconductor device designs. Overlay control must be performed on a run-to-run basis, i.e. at the end of a run when product characteristics are available, because they cannot be directly measured during a run. In this research a process model and a run-to-run control scheme was developed for overlay control, based on linear model predictive control, and successfully implemented in a commercial facility. Performance monitoring of the closed-loop process was also carried out.

Recursive least squares estimation for run-to-run control with metrology delay and its application to STI etch process

Run-to-run (RtR) control technology has received tremendous interest in semiconductor manufacturing. Exponentially weighted moving average (EWMA), double-EWMA, and internal model control (IMC) filters are recognized methods for online RtR estimation. In this paper, we consider recursive least squares (RLS) as an alternative for online estimation and RtR control. The relationship between EWMA-type and RLS-type estimates is analyzed and verified with simulations. Because measurement delay is almost inevitable in semiconductor manufacturing, we discuss and compare the performance of EWMA, RtR-IMC, and RLS controllers in handling measurement delay and measurement noise for processes with a deterministic drift. An ad hoc solution is proposed to handle measurement delay for processes with time-varying drifts. The results are illustrated through several simulations and a shallow trench isolation (STI) etch process as an industrial example.

Model-based control in microelectronics manufacturing

In order to achieve process control systems in semiconductor manufacturing that are able to maximize yield at minimum cost, an integrated approach that combines advanced control techniques and mathematical modeling with available online measurements is necessary. We have utilized a model predictive control approach for multivariate run-to-run control of chemical mechanical planarization (CMP), lithography, and rapid thermal processing reactors. Improvements due to advanced control have been quantified in actual fab operations.

Run-to-run control and state estimation in high-mix semiconductor manufacturing

Abstract In the modeling and control of semiconductor manufacturing, the control engineer must be aware of all influences on the performance of each process. Upstream processes may affect the wafer substrate in a manner that alters performance in downstream operations, and the context within which a process is run may fundamentally change the way the process behaves. Incorporating these influences into a control method ultimately leads to better predictability and improved control performance. Control threads are a way of incorporating these effects into the control of a process by partitioning historical data into groups within which the deterministic sources of variation are uniform. However, if there are many products, which require many threads to be defined, there may be insufficient data to model each thread. This multi-product–multi-tool manufacturing environment (“high-mix”) requires advanced methodologies based on state estimation and recursive least squares. Several such approaches are compared in this paper based on simulation models for a high-mix fab.

Lithography overlay controller formulation

Lithography overlay refers to the measurement of the alignment of successive patterns within the manufacture of semiconductor devices. Control of overlay has become of great importance in semiconductor manufacturing, as the tolerance for overlay error is continually shrinking in order to manufacture next-generation semiconductor products. Run-to-run control has become an attractive solution to many control problems within the industry, including overlay. The term run-to-run control refers to any automated procedure whereby recipe settings are updated between successive process runs in order to keep the process under control. The following discussion will present the formulation of such a controller by examining control of overlay. A brief introduction of overlay will be given, highlighting the control challenge overlay presents. A data management methodology that groups like processes together in order to improve controllability, referred to as control threads, will then be presented. Finally, a discussion of linear model predictive control will show its utility in feedback run-to-run control.

Recursive least squares estimation and its application to shallow trench isolation

In recent years, run-to-run (R2R) control technology has received tremendous interest in semiconductor manufacturing. One class of widely used run-to-run controllers is based on the exponentially weighted moving average (EWMA) statistics to estimate process deviations. Using an EWMA filter to smooth the control action on a linear process has been shown to provide good results in a number of applications. However, for a process with severe drifts, the EWMA controller is insufficient even when large weights are used. This problem becomes more severe when there is measurement delay, which is almost inevitable in semiconductor industry. In order to control drifting processes, a predictor-corrector controller (PCC) and a double EWMA controller have been developed. Chen and Guo (2001) show that both PCC and double-EWMA controller are in effect Integral-double-Integral (I-II) controllers, which are able to control drifting processes. However, since offset is often within the noise of the process, the second integrator can actually cause jittering. Besides, tuning the second filter is not as intuitive as a single EWMA filter. In this work, we look at an alternative way Recursive Least Squares (RLS), to estimate and control the drifting process. EWMA and double-EWMA are shown to be the least squares estimate for locally constant mean model and locally constant linear trend model. Then the recursive least squares with exponential factor is applied to shallow trench isolation etch process to predict the future etch rate. The etch process, which is a critical process in the flash memory manufacturing, is known to suffer from significant etch rate drift due to chamber seasoning. In order to handle the metrology delay, we propose a new time update scheme. RLS with the new time update method gives very good result. The estimate error variance is smaller than that from EWMA, and mean square error decrease more than 10% compared to that from EWMA.

RUN-TO-RUN CONTROL AND PERFORMANCE MONITORING OF OVERLAY IN SEMICONDUCTOR MANUFACTURING

Abstract In the manufacture of semiconductor products, overlay is one of the most critical design specifications. Overlay is the position of a pattern relative to underlying layers, and overlay control largely determines the minimum feature size that may be incorporated into semiconductor device designs. Overlay control must be performed on a run-to-run basis, i.e. at the end of a run when product characteristics are available because they cannot be directly measured during a run. In this research a process model and a run-to-run control scheme was developed for overlay control, based on linear model predictive control (LMPC), and successfully implemented in a commercial facility. Performance monitoring of the closed-loop process was also carried out.