Design of elliptically-vibrating ultrasonic actuator for nanocoining

• An elliptically-vibrating ultrasonic actuator was designed to form subwavelength features on a workpiece at 40 kHz.
• Using FEA, 2D resonant motion was achieved by matching two orthogonal vibration modes to occur at the same frequency.
• A closed-loop controller was designed and implemented to automatically track resonance and control vibrational motion of actuator.
• Indentations were performed and 500 nm pitch features were created at 40 kHz on a 360 brass workpiece.

Nanocoining is a method of rapidly creating a cylindrical mold surface covered with features smaller than the wavelength of light. This mold can then be used in a roll-to-roll process to make surfaces whose functionality depends on the wavelength of the illumination. The die replaces the typical diamond tool used to produce overlapping grooves for applications such as reflective signs. The die has a face area approximately 20 μm2 that has been patterned in an FIB. It is mounted on a 2D ultrasonic actuator and follows an elliptical path that matches the surface speed of the moving workpiece during the short contact time and creates approximately 6000 features per impact. The spacing of die indents is controlled by the speed of the diamond turning machine axes such that a small overlap exists from previous indents as the die spirals around and along the mold surface. Because the die is small, the indentations must occur rapidly to make nanocoining a feasible process. This work focuses on the design and control of a nominally 40 kHz, 2D resonant actuator that is suitable for this process. A controller to automatically track resonance is described to maintain the elliptical motion during indentation. Methods of tuning the behavior of the actuator and maintaining a constant indent depth are proposed. Finally, 500 nm pitch feature indents were created on a brass workpiece at 40 kHz and scanning electron microscope (SEM) images of the features are provided.