Dynamically balancing a flexure-based scan stage inside a scanning electron microscope

Delmic's SECOM platform integrates an optical microscope in a scanning electron microscope. A flexure based scan stage has been designed and produced to be used in the next generation of the SECOM platform. However, the reaction forces of the scan stage produce vibrations in the platform. These vibrations negatively affect the position accuracy and velocity stability of the scan stage, resulting in very poor image quality. To successfully implement the scan stage in the new platform, the errors must be reduced. The errors caused by the reaction forces of the scan stage in the SECOM platform are quantified by modelling, testing and calculations. Conventional methods to reduce errors do not offer a direct solution for the errors caused by the scan stage. Literature is reviewed in search of other disciplines that are capable of reducing reaction force related errors. Dynamic balancing, often used in linkages, can theoretically completely eliminate all reaction forces of a mechanism, and therefore remove the error source in the SECOM platform. The scan stage is redesigned in order to dynamically balance the mechanism. To use dynamic balancing is a challenge; no examples of macro-scale dynamically-balanced flexure mechanisms have been found in literature. Literature is also studied on the design of ultra-precision XY and XY$\Theta$ mechanisms. The most commonly used components are identified, and the optimal design configuration is concluded. The scan stage is designed according to the optimal design guidelines. The end effector of the scan stage is exact constrained, another novel feature when compared to the mechanisms found in literature. Measurements on the performance of dynamically balanced linkage mechanisms are not common, most designs found in literature are purely theoretical. The scanning mechanism is produced and the dynamic performance of the mechanism is verified. Subsequently, the reaction force of the mechanism is measured. Measurements show that the reaction force has successfully been reduced by a factor 50. The errors caused by such a force in the SECOM platform are within acceptable limits. The base of the mechanism was assumed to be infinitely stiff, an assumption commonly made in dynamic balancing literature. However, measurements also show that internal reaction forces of the mechanism cause significant deformations in the base, which can transfer a force. This potentially applies to dynamically balanced linkages as well. Consequently, an additional design guideline has been proposed for creating mechanisms that transmit no reaction forces to other systems or components: The displacements as a result of the inertial forces, and forces caused by the strain of the flexure elements of the mechanism, should be zero with respect to the center of mass of the base of the mechanism, at all mounting points and connection points of the mechanism to other systems. In this thesis, a dynamically balanced flexure-based mechanism is successfully designed and tested. No comparable mechanism has been found in literature, and therefore a paper has been written on the design of the mechanism. The work of this thesis shows that the principle of dynamic balancing can be successfully used in the design of flexure mechanisms. As a result, flexure mechanisms can potentially become cheaper, more compact and easier to implement in systems. This could make way for flexure mechanisms to be used in new technologies, where they are not seen today.

Subject

dynamic balancing
positioning mechanism
high accuracy
high bandwidth
flexure
compliant
scanning

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http://resolver.tudelft.nl/uuid:fbe78dd2-beeb-4d7f-9094-9e6a8c9154fd

Embargo date

2017-12-31

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Student theses

Document type

master thesis

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Thesis.pdf

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