Metasurface Fabrication by Cryogenic and Bosch Deep Reactive Ion Etching.

The research field of metasurfaces has attracted considerable attention in recent years due to its high potential to achieve flat, ultrathin optical devices of high performance. Metasurfaces, consisting of artificial patterns of subwavelength dimensions, often require fabrication techniques with high aspect ratios (HARs). Bosch and Cryogenic methods are the best etching candidates of industrial relevance towards the fabrication of these nanostructures. In this paper, we present the fabrication of Silicon (Si) metalenses by the UV-Nanoimprint Lithography method and cryogenic Deep Reactive Ion Etching (DRIE) process and compare the results with the same structures manufactured by Bosch DRIE both in terms of technological achievements and lens efficiencies. The Cryo- and Bosch-etched lenses attain efficiencies of around 39% at wavelength λ = 1.50 µm and λ = 1.45 µm against a theoretical level of around 61% (for Si pillars on a Si substrate), respectively, and process modifications are suggested towards raising the efficiencies further. Our results indicate that some sidewall surface roughness of the Bosch DRIE is acceptable in metalense fabrication, as even significant sidewall surface roughness in a non-optimized Bosch process yields reasonable efficiency levels.

Towards high-throughput large-area metalens fabrication using UV-nanoimprint lithography and Bosch deep reactive ion etching.

We demonstrate the fabrication of diffraction-limited dielectric metasurface lenses for NIR by the use of standard industrial high-throughput silicon processing techniques: UV nano imprint lithography (UV-NIL) combined with continuous reactive ion etching (RIE) and pulsed Bosch deep reactive ion etching (DRIE). As the research field of metasurfaces moves towards applications, these techniques are relevant as potential replacements of commonly used cost-intensive fabrication methods utilizing electron beam ithography. We show that washboard-type sidewall surface roughness arising from the Bosch DRIE process can be compensated for in the design of the metasurface, without deteriorating lens quality. Particular attention is given to fabrication challenges that must be overcome towards high-throughput production of relevance to commercial applications. Lens efficiencies are measured to be 25.5% and 29.2% at wavelengths λ = 1.55µm and λ = 1.31µm, respectively. A number of routes towards process optimization are proposed in relation to encountered challenges.

Modeling of micromachined silicon-polymer 2-2 composite matching layers for 15MHz ultrasound transducers.

Silicon-polymer composites fabricated by micromachining technology offer attractive properties for use as matching layers in high frequency ultrasound transducers. Understanding of the acoustic behavior of such composites is essential for using them as one of the layers in a multilayered transducer structure. This paper presents analytical and finite element models of the acoustic properties of silicon-polymer composites in 2-2 connectivity. Analytical calculations based on partial wave solutions are applied to identify the resonance modes and estimate effective acoustic material properties. Finite Element Method (FEM) simulations were used to investigate the interaction between the composite and the surrounding load medium, either a fluid or a solid, with emphasis on the acoustic impedance of the composite. Composites with lateral periods of 20, 40 and 80µm were fabricated and used as acoustic matching layers for air-backed transducers operating at 15MHz. These composites were characterized acoustically, and the results were compared with analytical calculations. The analytical model shows that at low to medium silicon volume fraction, the first lateral resonance in the silicon-polymer 2-2 composite is defined by the composite period, and this lateral resonant frequency is at least 1.2 times higher than that of a piezo-composite with the same polymer filler. FEM simulations showed that the effective acoustic impedance of the silicon-polymer composite varies with frequency, and that it also depends on the load material, especially whether this is a fluid or a solid. The estimated longitudinal sound velocities of the 20 and 40µm period composites match the results from analytical calculations within 2.7% and 2.6%, respectively. The effective acoustic impedances of the 20 and 40µm period composites were found to be 10% and 26% lower than the values from the analytical calculations. This difference is explained by the shear stiffness in the solid, which tends to even out the surface displacements of the composites.

Microfabricated 1-3 composite acoustic matching layers for 15 MHz transducers.

Medical ultrasound transducers require matching layers to couple energy from the piezoelectric ceramic into the tissue. Composites of type 0-3 are often used to obtain the desired acoustic impedances, but they introduce challenges at high frequencies, i.e. non-uniformity, attenuation, and dispersion. This paper presents novel acoustic matching layers made as silicon-polymer 1-3 composites, fabricated by deep reactive ion etch (DRIE). This fabrication method is well-established for high-volume production in the microtechnology industry. First estimates for the acoustic properties were found from the iso-strain theory, while the Finite Element Method (FEM) was employed for more accurate modeling. The composites were used as single matching layers in 15 MHz ultrasound transducers. Acoustic properties of the composite were estimated by fitting the electrical impedance measurements to the Mason model. Five composites were fabricated. All had period 16 µm, while the silicon width was varied to cover silicon volume fractions between 0.17 and 0.28. Silicon-on-Insulator (SOI) wafers were used to get a controlled etch stop against the buried oxide layer at a defined depth, resulting in composites with thickness 83 µm. A slight tapering of the silicon side walls was observed; their widths were 0.9 µm smaller at the bottom than at the top, corresponding to a tapering angle of 0.3°. Acoustic parameters estimated from electrical impedance measurements were lower than predicted from the iso-strain model, but fitted within 5% to FEM simulations. The deviation was explained by dispersion caused by the finite dimensions of the composite and by the tapered walls. Pulse-echo measurements on a transducer with silicon volume fraction 0.17 showed a two-way -6 dB relative bandwidth of 50%. The pulse-echo measurements agreed with predictions from the Mason model when using material parameter values estimated from electrical impedance measurements. The results show the feasibility of the fabrication method and the theoretical description. A next step would be to include these composites as one of several layers in an acoustic matching layer stack.