Using SiO₂ Hard Mask for Fabrication of Micro Fresnel Focusing Lens for Ultrasonic Ejectors

Abstract

The fabrication of Fresnel focusing lenses operating at frequencies of 100 and 200 MHz was investigated in order to enhance the focusing efficiency of ultrasonic energy. The effects of process parameters on the four-level Fresnel lens profiles were discussed to find a most feasible fabrication procedure through these experiments. The quality of Fresnel lenses was improved when two-and three-mask processes using SiO2 film as the hard mask were employed. Besides, a better side-wall profile of Fresnel lens was obtained by using the three-mask process as compared to the two-mask one.

Phase-Optimized Multi-Step Phase Acoustic Metasurfaces for Arbitrary Multifocal Beamforming

“…Four steps with different heights represent phase difference distributions of 0, π /2, π , and 3 π /2, respectively. The ring radii r n can be expressed as [ 34 ] where F is the focal length from the piezoelectric element, λ is wavelength in the propagation medium (water: sound speed: 1490 m/s, density: 1000 kg/m 3 ), and N is 4, indicating four-step phase distribution. To achieve π /2 phase shift, as shown in Figure 1 c, the longitudinal step h of the lens can be calculated as where f is the operating frequency of the ultrasonic transducer, and v w and v p are the sound speeds of water and lens, respectively.…”

“…Four steps with different heights represent phase difference distributions of 0, π/2, π, and 3π/2, respectively. The ring radii r n can be expressed as [34]…”