Designing a piezoelectric wafer stage for electron beam inspection systems
Ron de Bruijn defended his PhD thesis at the Department of Mechanical Engineering on November 27th.
Electron beam inspection systems are vital in the semiconductor industry, enabling precise nanoscale measurements and defect detection. These systems scan a wafer鈥檚 surface with a focused electron beam, producing high-resolution images. Their accuracy, however, is highly sensitive to electromagnetic interference, making actuator selection critical. Conventional Lorentz actuators provide smooth motion but generate residual magnetic fields, which require expensive shielding. In his PhD research, Ron de Bruijn investigates piezoelectric actuators as an alternative that avoids electromagnetic interference. While promising, their inherent stiffness makes them prone to transmitting disturbances. To overcome this limitation, De Bruijn proposes a novel wafer stage architecture.
Instead of relying on electromagnetic actuators for both coarse and fine positioning, Ron de Bruijn鈥檚 design combines piezoelectric actuators with a variable stiffness device. The device is designed to be stiff during acceleration phases, allowing the wafer table to be driven directly by the long-stroke stage. During the imaging phase, the device switches to a compliant state to significantly reduce the transmission of disturbances from the long-stroke stage to the wafer table. An intermediate body is incorporated to improve actuator efficiency by serving both as a balance mass and as a metrology reference for precise position measurement.
Material and design
The variable stiffness device is based on viscoelastic rubber materials, chosen for their near-incompressibility. This property allows a large variation in stiffness. By tuning the device geometry, De Bruijn achieved a stiffness ratio of approximately 4000 between the compliant and stiff states, while maintaining reduced wear at the contact interface.
Control strategy
A dedicated control strategy was developed to handle the changing system dynamics introduced by the stiffness switching. The long-stroke stage controller tracks its reference position and remains stable across different stiffness states. The wafer table feedback controller is inactive during acceleration, when the device is in a stiff state, and activated during image acquisition. During acceleration, micrometer-level accuracy is sufficient to allow fast settling after switching. The feedback controller is re-enabled once the device becomes sufficiently compliant to ensure stable control.
Simulations and experimental studies
The concept was validated through comprehensive simulations and experimental studies. A dedicated test setup was upgraded to integrate the variable stiffness device and piezoelectric wafer table actuation. Experimental validation included comparing dynamic models with frequency-domain identifications and evaluating both stiffness characteristics and switching-induced errors. An iterative learning control algorithm was implemented to minimize these switching errors. Performance experiments with white-noise disturbances to simulate realistic long-stroke servo behavior demonstrated an initial settling time of 20 ms and a steady-state servo error of about 60 nm. By implementing a bumpless transfer control scheme, the settling time was further improved to 5 ms. When motion-induced inertial forces were introduced, the system settled within 15 ms, consistent with the expected order of magnitude.
Conceptual design
Alongside modeling and experiments, a conceptual full-scale design of the proposed piezoelectric-actuated wafer stage was developed for integration into electron beam inspection systems. This includes the kinematic actuation layout, implementation of the variable stiffness device, and a magnetically clean long-stroke drive concept. To eliminate magnetic fields inside the process chamber, an alternative long-stroke actuation system is proposed, based on a four-arm planar parallel manipulator with electromagnetic actuation located outside the vacuum chamber. Ferrofluidic seals act as a barrier between the vacuum and atmospheric environment.
The insights from this research pave the way for developing a functional six-degree-of-freedom prototype of the concept. Such a prototype would allow further experimental evaluation and refinement, bringing magnetically clean, high-precision wafer stages closer to practical application in electron beam inspection systems.
This research is part of a project from .
Title of PhD thesis: . Supervisors: Prof. Hans Vermeulen, Prof. Maarten Steinbuch and Dr. Jeroen van de Wijdeven.