Roughness Suppression in Electrochemical Nanoimprinting of Si for Applications in Silicon Photonics

Abstract

Metal‐assisted electrochemical nanoimprinting (Mac‐Imprint) scales the fabrication of micro‐ and nanoscale 3D freeform geometries in silicon and holds the promise to enable novel chip‐scale optics operating at the near‐infrared spectrum. However, Mac‐Imprint of silicon concomitantly generates mesoscale roughness (e.g., protrusion size ≈45 nm) creating prohibitive levels of light scattering. This arises from the requirement to coat stamps with nanoporous gold catalyst that, while sustaining etchant diffusion, imprints its pores (e.g., average diameter ≈42 nm) onto silicon. In this work, roughness is reduced to sub‐10 nm levels, which is in par with plasma etching, by decreasing pore size of the catalyst via dealloying in far‐from equilibrium conditions. At this level, single‐digit nanometric details such as grain‐boundary grooves of the catalyst are imprinted and attributed to the resolution limit of Mac‐Imprint, which is argued to be twice the Debye length (i.e., 1.7 nm)—a finding that broadly applies to metal‐assisted chemical etching. Last, Mac‐Imprint is employed to produce single‐mode rib‐waveguides on pre‐patterned silicon‐on‐insulator wafers with root‐mean‐square line‐edge roughness less than 10 nm while providing depth uniformity (i.e., 42.9 ± 5.5 nm), and limited levels of silicon defect formation (e.g., Raman peak shift < 0.1 cm−1) and sidewall scattering.

Metal-assisted chemical etching beyond Si: applications to III–V compounds and wide-bandgap semiconductors

“…Advances in in situ analysis to more precisely determine reaction species would certainly benefit the MacEtch community. The recent development of two-dimensional materials, 193,205,207,213 oxides, 25 and nitrides 209,214 as catalysts, one-pot MacEtch, 66,82 and MacImprint 36,215 techniques are promising developments on these issues. By the same token, the advantages of MacEtch have the potential to greatly innovate mainstay nanofabrication techniques, by improving processing throughput, reducing fabrication complexity, avoiding surface damage, and lowering costs.…”

High aspect ratio arrays of Si nano-pillars using displacement Talbot lithography and gas-MacEtch

“…In contrast, MacEtch allows to fabricate hierarchical features [45,46] with very sharp profile. MacEtch has been successfully demonstrated in combination with electron beam [47,48], UV [44,[49][50][51], flow-enabled self-assembly [52], tip-based [53], electrochemical nanoimprint [54] and chemisorption-assisted transfer printing [55] lithography. We recently reported a new method of MacEtch in gas phase (gas-MacEtch) [45], which combines the extremely high aspect ratio capability of this technique with the use of gas reactants to reduce the nanostructures stiction due to capillary effects during drying after wet etching [56].…”

Digital and Gradient Refractive Index Planar Optics by Nanoimprinting Mesoporous Silicon

“…Nanostructured high refractive index materials offer a powerful means for tailoring the optical characteristics of planar optical components used in flat-optics, [1] metamaterials, [2] integrated photonics, [3][4][5] and transformation optics. [6,7] For many planar optical devices ranging from diffractive elements [8] and metalenses, [9][10][11] to optical cloaks, [7] waveguide-based devices, [3,4,12,13] and more, an ideal design solution is to engineer a sub-wavelength spacing between the dielectric elements, or in other words to construct a sub-wavelength grating (SWG) metama-demonstrated in infrared glass-ceramics and nanocomposites by local modification with laser, [25] thermal, [23,24] or photothermal treatments, [26] with an achievable index contrast up to Δn = 0.1.…”