Cr etchant solution

The silicon (Si) metasurfaces are fabricated with electron-beam lithography and reactive ion etching. A 600 nm thick amorphous Si film is deposited on a glass substrate using an electron beam evaporator (SKE_A_75). Then, a 100 nm thick electron-beam resist (MicroChem PMMA [polymethyl methacrylate] A2) is spin-coated onto the Si film and patterned with the electron beam writer (Raith E-line, 30 kV). After development in a MIBK & IPA (1:3) solution, 15 nm thick Chromium (Cr) is deposited on the sample using electron beam evaporation (SKE_A_75) and the inverse nano-pattern is transferred to the Cr layer by a lift-off process in a remover PG (Micro Chem). By etching the Si with Sulfur hexafluoride (SF6) and Fluoroform (CHF3) in a flow of 5 sccm and 50 sccm, respectively, the nano-patterns are transferred to the Si membrane. Finally, the Cr mask is removed by immersing the samples in a Cr etchant solution (Aldrich Chemistry) for 30 minutes.

The Si metasurfaces were fabricated by a combined process of electron-beam lithography and reactive ion etching. The 230 nm single-crystal silicon film on a sapphire wafer was cleaned with acetone and coated with 100 nm electron-beam resist polymethyl methacrylate. The sample was baked at 180 °C for 1 h and then patterned by electron-beam writer (Raith E-line) with a dose 90 µC/cm² under an acceleration voltage 30 kV. After developing with MIBK&IPA (1:3) solution, 15 nm Cr was deposited on the sample using E-beam evaporation (SKE_A_75), and Cr hard mask was realized via a liftoff process in remover PG solution (Micro Chem) for 24 h. The pattern was transferred to the Si through reactive ion etching with SF₆ and CHF₃ in Oxford Plasmalab System 100. The vacuum degree was 10⁻⁵ and the gas flow was 5 and 50 sccm for SF6 and CHF3, respectively. Finally, the Si metasurfaces were obtained by removing the Cr mask in Cr etchant solution (Aldrich Chemistry) for 30 min.

Firstly, the 13 nm indium tin oxide (ITO) on glass substrate is ultrasonically cleaned. The 800 nm TiO2 film is deposited on the ITO coated glass substrate by using an electron beam (Ebeam) evaporator (SKE_A_75). The refractive index, extinction coefficient, and thickness are measured by ellipsometer. The 100 nm electron-beam resist (PMMA) is spin-coated at 4000 rpm on the film and baked at 180 °C for 1 h. The resist is patterned by electron-beam lithography (Raith E-line) at a dose of 90 μC/cm² at an accelerating voltage of 30 keV. After immersing in MIBK&IPA (1:3) solution for 30 s, a reverse pattern is generated on the surface. A layer of 23 nm chromium (Cr) is deposited on the sample using Ebeam evaporation. After the samples is soaked in remover PG solution (Micro Chem) for 12 h, the pattern was then transferred to the TiO2 film by reactive ion etching (Oxford Plasmalab System 100). Finally, the Cr hard mask was removed by soaking in a Cr etchant solution (Aldrich Chemistry) for 10 min.