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Method for robust run-to-run control for a lithographic process with a changing working zone and immeasurable disturbances

IP.com Disclosure Number: IPCOM000029752D
Publication Date: 2004-Jul-09
Document File: 5 page(s) / 123K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for stable and robust run-to-run control for a lithographic process with a changing working zone and immeasurable disturbances. Benefits include improved functionality, improved reliability, and improved design flexibility.

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Method for robust run-to-run control for a lithographic process with a changing working zone and immeasurable disturbances

Disclosed is a method for stable and robust run-to-run control for a lithographic process with a changing working zone and immeasurable disturbances. Benefits include improved functionality, improved reliability, and improved design flexibility.

Background

              Complicated run-to-run control problems occur in lithographic critical dimension control. In the semiconductor manufacturing process, problems include the following:

•             Changing operating zone: For example, critical dimension (CD) is ideally a fixed function of dose and focus. But with many factors affecting the process, the operating zone changes. Thin-film thickness is one of the major factors.

•             Unknown disturbances: Unknown disturbances are immeasurable factors, such as resist uniformity and stepper lens heating. The disturbance is random and may or may not have Gaussian distributions.

•             Process drift: The process may drift in an unpredictable way. A fixed model from offline data cannot capture the variations in real time.

•             Non-linearity: CD has nonlinear relationship with focus when an exposure dose is fixed. A simple proportional focus control based on CD error and a fixed slope does not work for the nonlinear case.

              Conventionally, the problem is solved by assuming the working zone is fixed, the disturbance is white noise, and the system is linear. Then, a controller with the exponential weighted moving average (EWMA) can be applied to control the critical dimension in lithography. Under these assumptions, the EWMA controller may work for a short time period before a shutdown is required for a reset. In summary, the control problem is not solved (see Figure 1).

General description

              The disclosed method is a combination of two controllers for the lithographic critical dimension with the capability of recognizing the changing working zone and stabilizing the process under immeasurable disturbances. The key elements of the method include:

•             Automatic controller for the lithographic CD run-to-run control

•             Controller tolerates measurable disturbances, such as thin-film thickness and immeasurable disturbances in real-time, including wafer flatness, resist uniformity, and stepper lens heating

Advantages

The disclosed method provides advantages, including:

•             Improved functionality due to providing a successful control methodology for critical dimension problems in the semiconductor manufacturing process

•             Improved reliability due to providing system stabilization during disturbances

•             Improved reliability due to compensating for process drift/shift

•             Improved design flexibility due to being applicable to linear and nonlinear systems

•             Improved design flexibility due to being applicable to any semiconductor process

Detail...