Multi-step plasma etching of high aspect ratio silicon nanostructures for metalens fabrication

Choi H., Kim J., Kim W., Seong J., Park C., Choi M., Kim N., Ha J., Qiu C., Rho J., Lee H.

AbstractNanoimprint lithography (NIL) has attracted attention recently as a promising fabrication method for dielectric metalenses owing to its low cost and high throughput, however, high aspect ratio (HAR) nanostructures are required to manipulate the full 2π phase of light. Conventional NIL using a hard-polydimethylsiloxane (h-PDMS) mold inevitably incurs shear stress on the nanostructures which is inversely proportional to the surface area parallel to the direction of detachment. Therefore, HAR structures are subjected to larger shear stresses, causing structural failure. Herein, we propose a novel wet etching NIL method with no detachment process to fabricate flawless HAR metalenses. The water-soluble replica mold is fabricated with polyvinyl alcohol (PVA) which is simpler than an h-PDMS mold, and the flexibility of the PVA mold is suitable for direct printing as its high tensile modulus allows high-resolution patterning of HAR metalenses. The diffraction-limited focusing of the printed metalenses demonstrates that it operates as an ideal lens in the visible regime. This method can potentially be used for manufacturing various nanophotonic devices that require HAR nanostructures at low cost and high throughput, facilitating commercialization.

Shi Z., Jefimovs K., Stampanoni M., Romano L.

Structuring Si in arrays of vertical high aspect ratio pillars, ranging from nanoscale to macroscale feature dimensions, is essential for producing functional interfaces for many applications. Arrays of silicon 3D nanostructures are needed to realize photonic and phononic crystals, waveguides, metalenses, X-ray wavefront sensors, detectors. In particular, arrays of Si nanopillars are used as bio-interfaces in neural activity recording, cell culture, microfluidics, sensing and on-chip manipulation. Here, we demonstrate a strategy for realizing arrays of protruding sharp Si nanopillars, using displacement Talbot lithography combined with metal-assisted chemical etching (MacEtch) in gas phase. Such combination enables reliable and low cost pathway for fabrication of ordered nanopillars arrays on large scale. With the double exposure of a linear grating mask in orthogonal orientations and the lift-off technique, we realized a catalyst pattern of holes in a Pt thin film with a period of 1 μm and hole diameter in the range of 100–250 nm. MacEtch in gas phase by using vapor HF and oxygen from air allows to etch arrays of protruding Si nanopillars 200 nm-thick and aspect ratio in the range of 200 (pillar height/width) with an etching rate up to 1 μm/min. Gas-MacEtch has the advantage of no capillary stiction, no ion beam damage of the Si substrate, nanometric resolution and high fidelity of pattern transfer. In combination with controlled uniformity of feature size on large area, spatial frequency doubling and high resolution of Talbot lithography, the proposed method is an easy-to-scale-up processing that can support the fabrication of Si pillars arrays for many valuable applications both at micro and nano-scale.

Zhu Y., Wang W., Yu F., Liu Q., Guo Z., Li G., Chen P., Lu W.

Metalenses play an important role in optoelectronic integrated devices, given their advantages in miniaturization and integration. Due to its high aspect ratio subwavelength structure, fabricating metalenses requires a high-level dry etching technology. Consequently, structure deformation of the metalens will exist if the etching process of the material is not mature enough, which will impair the metalens’ performance. In this paper, a polarization-independent InP dielectric metalens is designed to focus the incident light from air into the substrate, which is used for monolithically integrating with the InGaAs/InP photodetector in the future. Subsequently, with the simulation method, we investigated the impact of the structure deformation on the metalens’ performance, which was found in our InP dry etching process development. We have found that the sidewall slope and aspect ratio-dependent etching effect greatly impaired the focusing efficiency because of the phase modulation deviation. To solve this problem, we proposed a manufacturing-tolerant design method, which effectively improved the performance of the device with structural deformation. Our work is instructive for developing metalenses and can accelerate their integration application.

Huff M.

This paper reviews the recent advances in reaction-ion etching (RIE) for application in high-aspect-ratio microfabrication. High-aspect-ratio etching of materials used in micro- and nanofabrication has become a very important enabling technology particularly for bulk micromachining applications, but increasingly also for mainstream integrated circuit technology such as three-dimensional multi-functional systems integration. The characteristics of traditional RIE allow for high levels of anisotropy compared to competing technologies, which is important in microsystems device fabrication for a number of reasons, primarily because it allows the resultant device dimensions to be more accurately and precisely controlled. This directly leads to a reduction in development costs as well as improved production yields. Nevertheless, traditional RIE was limited to moderate etch depths (e.g., a few microns). More recent developments in newer RIE methods and equipment have enabled considerably deeper etches and higher aspect ratios compared to traditional RIE methods and have revolutionized bulk micromachining technologies. The most widely known of these technologies is called the inductively-coupled plasma (ICP) deep reactive ion etching (DRIE) and this has become a mainstay for development and production of silicon-based micro- and nano-machined devices. This paper will review deep high-aspect-ratio reactive ion etching technologies for silicon, fused silica (quartz), glass, silicon carbide, compound semiconductors and piezoelectric materials.

Baracu A.M., Dirdal C.A., Avram A.M., Dinescu A., Muller R., Jensen G.U., Thrane P.C., Angelskår H.

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.

Ekinci H., Jahed N.M., Soltani M., Cui B.

Chromium and its oxides have been playing a vital role in the fabrication of micro- and nano-scale structures in numerous applications for several decades. Controllable, robust and anisotropically dry-etched hard masks and their optimal etch recipes are required in state-of-the-art device fabrication techniques. In terms of manufacturability and repeatability, a mechanistic understanding of the plasma-etching process of chromium oxide (Cr 2 O 3 ) is necessary for its adoption as a hard mask. We present a systematic investigation of plasma etching of chromium oxide films via an inductively coupled plasma-reactive ion etching (ICP-RIE) system in nanoscale. The effects of plasma composition, ICP source power and HF platen power on the etch rate, sidewall profile, surface morphology, and dc-bias have been methodically investigated. We paid particular attention to studying how oxygen content can be used to control the etch profile of nano trenches using chlorine/oxygen gas mixtures, including extremes of very low and very high oxygen content. It was found that chromium oxide etch mechanisms are dependent strongly on the oxygen level. We achieved desirable vertical sidewalls with reasonable etch rates when the oxygen content is in the range 10-40% in the plasma. Oxygen content below 10% resulted in positively tapered etch profiles with low etch rates. On the other hand, bowl-like etch profiles with undercut formation was observed at high oxygen content above 40%, caused by re-emission of the reactive species at this regime. As a hard mask material, patterning Cr 2 O 3 films compared to Cr metal is advantageous in terms of etch uniformity and reproducibility. Contrary to Cr, Cr 2 O 3 is not as sensitive to chamber wall conditions.

Chen W.T., Zhu A.Y., Capasso F.

Control over the dispersion of the refractive index is essential to the performance of most modern optical systems. These range from laboratory microscopes to optical fibres and even consumer products, such as photography cameras. Conventional methods of engineering optical dispersion are based on altering material composition, but this process is time-consuming and difficult, and the resulting optical performance is often limited to a certain bandwidth. Recent advances in nanofabrication have led to high-quality metasurfaces with the potential to perform at a level comparable to their state-of-the-art refractive counterparts. In this Review, we introduce the underlying physical principles of metasurface optical elements (with a focus on metalenses) and, drawing on various works in the literature, discuss how their constituent nanostructures can be designed with a highly customizable effective index of refraction that incorporates both phase and dispersion engineering. These metasurfaces can serve as an essential component for achromatic optics with unprecedented levels of performance across a broad bandwidth or provide highly customized, engineered chromatic behaviour in instruments such as miniature aberration-corrected spectrometers. We identify some key areas in which these achromatic or dispersion-engineered metasurface optical elements could be useful and highlight some future challenges, as well as promising ways to overcome them. Flat metasurface optics provides an emerging platform for combining semiconductor foundry methods of manufacturing and assembling with nanophotonics to produce high-end and multifunctional optical elements. This Review highlights the design of metasurfaces, recent advances in the field and initial promising applications.

Zhou Y., Hu T., Li Y., Li N., Dong Y., Li D., Fu Y.H., Zhong Q., Xu Z., Zhu S., Lin Q., Singh N.

Metalenses based on dielectric nanostructures operating in the transmission mode are of great practical significances. As large-area, multifunctional metalenses are increasingly demanded at higher volumes nowadays, the quality and efficiency of the nanofabrication processes for these lenses are made even more important. Yet, no fabrication is without defects or errors. By using finite-difference time-domain (FDTD) calculation method, we study, in simulation, a set of common fabrication-induced errors such as inclined sidewall, critical dimension (CD) bias and process defects, and analyze their influences on the optical performances of two types of metalenses - one made of amorphous silicon and the other of silicon nitride. The study concludes that the lens behavior generally relies on the resonant nature of the nanostructure array. While most types of defects discussed in this work linearly decrease the focusing efficiency, a few would also influence the quality of the focal spot. Potential solutions to mitigate the effect of the process-induced defects are also investigated. The study provides the particular knowledge to understand the actual metalens performance when fabricated.

Dirdal C.A., Jensen G.U., Angelskår H., Vaagen Thrane P.C., Gjessing J., Ordnung D.A.

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.

Kim S., Yang K., Shin Y., Kim K., Kim D., Lee J.Y., Kim Y., Yeom G.

The etch characteristics of Si and TiO2 nanostructures for optical devices were investigated using pulse biased inductively coupled plasmas (ICP) with SF6/C4F8/Ar and BCl3/Ar, respectively, and the results were compared with those etched using continuous wave (CW) biased ICP. By using pulse biasing compared to CW biasing in the etching of the line/pillar nanostructures with various aspect ratios, there was a reduction of the aspect ratio dependent etching (ARDE) and therefore, uniform etch depths for nanostructures with different pattern widths, as well as the improvement of the etch profiles without any notching, were obtained not only for silicon nanostructures but also for TiO2 nanostructures. The investigation has determined that the improvement of etch profiles and reduced ARDE effect when using pulse biasing are related to the decreased surface charging caused by neutralization of the surface and the improved radical adsorption (or etch byproduct removal) on the etched surfaces during the pulse-off period for pulse biasing compared to CW biasing.

Hu T., Zhong Q., Li N., Dong Y., Xu Z., Fu Y.H., Li D., Bliznetsov V., Zhou Y., Lai K.H., Lin Q., Zhu S., Singh N.

AbstractMetalenses made of artificial sub-wavelength nanostructures have shown the capability of light focusing and imaging with a miniaturized size. Here, we report the demonstration of mass-producible amorphous silicon metalenses on a 12-inch glass wafer via the complementary metal-oxide-semiconductor compatible process. The measured numerical aperture of the fabricated metalens is 0.496 with a focusing spot size of 1.26 μm at the wavelength of 940 nm. The metalens is applied in an imaging system to test the imaging resolution. The minimum bar of the resolution chart with a width of 2.19 μm is clearly observed. Furthermore, the same system demonstrates the imaging of a fingerprint, and proofs the concept of using metalens array to reduce the system size for future compact consumer electronics.

Romano L., Kagias M., Vila-Comamala J., Jefimovs K., Tseng L., Guzenko V.A., Stampanoni M.

Gas-MacEtch of Si with a Pt catalyst allows vertical etching nanostructures with an extreme aspect ratio up to 10 000 : 1.

Dey R.K., Ekinci H., Cui B.

High aspect ratio silicon structures have gained significant interest due to their vast applications. Minimal lateral etch under the mask is essential to achieve such high aspect ratio structures. Previously, the authors reported that chromium oxide is better than metallic chromium as a hard mask for silicon etching in terms of etch rate and selectivity to resist during mask structure fabrication. Here, it is reported that a metal oxide etch mask also gives less lateral etch than a metal etch mask. Following mask structure fabrication by electron beam lithography and lift-off, silicon was etched using a nonswitching (i.e., SF6 and C4F8 gases simultaneously injected into a chamber) pseudo-Bosch process. The amount of lateral etching right underneath the mask is less (roughly half) for Cr2O3 and Al2O3 masks than Cr or Al masks. One plausible explanation for the difference is the metal-assisted plasma etching effect where the metal catalyzes the chemical reaction by injecting holes into the silicon in contact. It is also reported that a higher bias power leads to less undercut than a lower one, due to increased and more directional physical bombardment by ions.High aspect ratio silicon structures have gained significant interest due to their vast applications. Minimal lateral etch under the mask is essential to achieve such high aspect ratio structures. Previously, the authors reported that chromium oxide is better than metallic chromium as a hard mask for silicon etching in terms of etch rate and selectivity to resist during mask structure fabrication. Here, it is reported that a metal oxide etch mask also gives less lateral etch than a metal etch mask. Following mask structure fabrication by electron beam lithography and lift-off, silicon was etched using a nonswitching (i.e., SF6 and C4F8 gases simultaneously injected into a chamber) pseudo-Bosch process. The amount of lateral etching right underneath the mask is less (roughly half) for Cr2O3 and Al2O3 masks than Cr or Al masks. One plausible explanation for the difference is the metal-assisted plasma etching effect where the metal catalyzes the chemical reaction by injecting holes into the silicon in contac...

Laermer F., Urban A.

Ha J., Ndao A., Hsu L., Park J., Kante B.

Conventional optical components have been proposed to realize high-quality line focusing with uniform intensity distribution such as cylindrical lenses, segmented wedge-arrays, or a combination of prisms and spherical mirrors. Numerous factors such as the manufacturing tolerances or the need for precise alignment of conventional lenses cause wave front aberrations that impact the performance of optical systems. These aforementioned limitations affect the uniformity of the intensity distribution and the intercept factor of lenses. Here, we experimentally demonstrate an integrable planar dielectric cylindrical lens made of titanium dioxide for uniform line focusing and discuss the sensitivity of its performance to fabrication imperfections originating from non-ideal geometrical parameters. The lens has a numerical aperture of 0.247, an intercept factor of 0.85, and an efficiency of 79% at 800 nm.

Shen L., GE S., zhang J., Chen R., Li T., Liu W.

Multi-level diffractive lenses (MDLs), known for their remarkable broadband achromatic properties, emerge as a popular research topic for lightweight optical systems. As advancements in the optimization design framework continue, the distortion of the MDL feature structure (ring-bands) caused by manufacturing accuracy becomes a critical factor restricting their practical application. This research refines the functional relationships associated with the MDL feature structure and presents an innovative analytical method based on differential profile parsing (DPP) to efficiently and accurately investigate the performance loss caused by the distortion of the MDL feature structure. As a result, investigations into lens performance loss are undertaken for seven types of distortions in three categories, which arise from several typical MDL manufacturing processes. By using the DPP method, this study provides insights into the manufacturing tolerances of the MDL and general recommendations for process optimization. The final optical characterization largely verifies that the proposed theory provides valuable guidance in practical manufacturing applications, illustrating that reducing the distortion degree of the MDL feature structure dramatically improves MDL optical performance.

Nasir F., Pan A., Cui B.

Moothedath A., Ren Z.

Fetisenkova K., Melnikov A., Kuzmenko V., Miakonkikh A., Rogozhin A., Tatarintsev A., Glaz O., Kiselevsky V.

The selectivity of the reactive ion etching of silicon using a negative electron resist AR-N 7520 mask was investigated. The selectivity dependencies on the fraction of SF6 in the feeding gas and bias voltage were obtained. To understand the kinetics of passivation film formation and etching, the type and concentration of neutral particles were evaluated and identified using plasma optical emission spectroscopy. Electron temperature and electron density were measured by the Langmuir probe method to interpret the optical emission spectroscopy data. A high etching selectivity of 8.0 ± 1.8 was obtained for the etching process. The optimum electron beam exposure dose for defining the mask was 8200 pC/m at 30 keV.