A piezoelectric wafer stage: for electron beam inspection systems

Electron beam inspection system play a crucial role in the semiconductor industry, enabling precise inspection of semiconductor structures by measuring dimensions and detecting surface defects. These inspection systems operate by scanning the surface with a focused electron beam, generating secondary and backscattered electrons that are captured by a detector to form high-resolution images. However, electron beams are highly sensitive to electromagnetic interference, requiring shielding from the Earth's magnetic field and from electromagnetic fields generated by Lorentz actuators. Although Lorentz actuators offer good linearity and minimal position dependency, residual stray fields degrade performance and the required extensive magnetic shielding significantly increases system complexity and cost. This thesis explores piezoelectric actuators as an alternative to Lorentz actuators for accurately positioning the wafer in electron beam inspection systems. Piezoelectric actuators offer high force density, low power dissipation, and eliminate electromagnetic interference with the electron beam. However, their inherent high stiffness leads to high transmissibility of external disturbances. To address this challenge, an alternative wafer stage architecture is proposed. Existing wafer stages are based on electromagnetically actuated positioning systems both for large-range motion and for fine positioning. In contrast, the proposed design incorporates a variable stiffness device in series with the piezoelectric actuators. This device remains stiff during acceleration, allowing the wafer table to be driven directly by the long-stroke stage. For the accurate phase, during image acquisition, the variable stiffness device is switched to a compliant state, to significantly reduce transmissibility of disturbances from the long-stroke stage to the wafer table. An intermediate body is introduced to enhance actuator efficiency by serving as a balance mass and to provide a metrology reference for position measurement. The variable stiffness device is based on viscoelastic rubbery materials, chosen for the nearly volumetric incompressibility, enabling large stiffness variation. By adjusting the geometry, a stiffness ratio of 4000 can be attained between compliant and stiff state, with reduced wear at the contact interface. A novel control strategy has been developed to be able to cope with varying system dynamics. The long stroke stage controller tracks the setpoint and remains stable despite the changing dynamics. The wafer table feedback controller, however, is inactive during acceleration (when the variable stiffness device is stiff) and is enabled for image acquisition. During acceleration, the accuracy of the wafer table position is not very critical, limited to several micrometers to enable fast settling after switching. The wafer table feedback controller is enabled during the stiffness transition, when the variable stiffness device is compliant enough to guarantee stability. The proposed concept is validated through both extensive simulations and experiments. An existing test setup is upgraded to be able to incorporate a variable stiffness device and piezoelectric wafer table actuation. Experimental validation includes comparing dynamical models to frequency-domain identifications and assessing stiffness values of the variable stiffness device and switching-induced errors. An iterative learning control algorithm is developed to minimize the switching-induced errors. Performance experiments, which include white noise disturbances to create realistic long-stroke servo errors, reveal an initial settling time of 20 ms and a servo error of about 60 nm, which has been further improved to 5 ms by implementing bumpless transfer control. Introducing not only disturbances but incorporating inertial forces due to a motion setpoint, the system settles in 15 ms, within the expected order of magnitude. In parallel to the modeling and the experimental validation, a conceptual design of the proposed piezoelectric-actuated wafer stage is developed for application in electron beam inspection systems, including kinematic wafer table actuation and variable stiffness device implementation. To eliminate magnetic fields inside the process chamber, an alternative long-stroke method is proposed with electromagnetic actuation outside the process chamber. This concept is based on a four-arm planar parallel manipulator concepts for reasons of workspace, symmetry and reduced contamination and complexity. Ferrofluidic seals are included as barrier between the vacuum and atmospheric pressure. As a next step, it is proposed to realize a functional model of the described concept in six degrees of freedom and to investigate it experimentally.