Real-time contactless vibration detection in lithographic systems

In this project the use of Nearfield Acoustic Holography for the accurate and contactless measuring of structural vibrations is investigated. It is the aim to improve the measurement accuracy by incorporating a radiation model of the object of interest and its environment,  together with an optimal positioning of the acoustic sensors.

PhD Candidate: Elise Moers
Supervisor: Ines Lopez-Arteaga, prof.dr. Henk Nijmeijer
Project funding: This work is supported by Point-One, an open association supported by the Dutch Ministry of Economic Affairs.
Project Period: September 2011 - August 2015

Due to the demand of increasing  throughput in lithographic machines, moving parts in the new generation machines are flexible. However to guarantee sub-nanometer accuracy in the positioning of the stage, flexural vibration modes are detected and controlled such that the excited modes are suppressed. As an alternative to conventional sensors, the use of Near-field Acoustic Holography (NAH)  to detect the vibration modes will be investigated and improved.

Planar Near field acoustic holography (PNAH) is a method to identify sound sources. We focus on a Fourier-based implementation in which measured acoustic pressure at one plane is transformed to structural vibrations in another plane.  The transformation can be split in several steps, which is illustrated in Figure 1.

In order to reach the accuracy requirements, a dedicated model of the radiation of plate-like structures is needed. The PhD project focuses on finding such model in reflective environment  and further use it to tailor-made an inverse propagator for PNAH. Also we investigate the accuracy limits of Fourier-based PNAH.