Table 1.
Summary of the fabrication methods for solid-state nanopores.
| Method | Material | Minimum Diameter | Inner Morphology | Fabrication Scale | Controllability/Reproducibility | Manufacturing Cost | Manufacturing Time/Rate |
|---|---|---|---|---|---|---|---|
| Focused ion/electron beam (FIB/FEB) drilling | SiN [16,42,43,44,45,54], SiO2 [46], Metallic and metal oxide [24,47,110,111], 2D materials [56,112,113,114] | 1.3 nm [54] | Cylindrical/Hourglass | One at a time | Good controllability (nanometer precision) | High (requires FIB or TEM) | 0.2 s for one pore [45]; hours for a high-density array |
| Dielectric breakdown | SiN [59,60], SiO2 [63], HfO2 [64] | <1 nm [60] | Cylindrical | One/multiple at a time | Poor controllability over the amount of nanopores | Low (requires a voltage source and electrolyte solutions) | ~ 1 h for one or multiple pores [60] |
| Metal-assisted chemical etching (MaCE) | Si [74,76,77,84] | 50 nm [84] | Conical | Array (pore density of 109 cm−2) [84] | Poor distribution in the pore size and location | Middle (requires heavy metal particles, HF/H2O2 solutions) | 30 min to etch 30 µm-thick Si, and 24 h to etch 500 µm-thick Si [84] |
| Electrochemical anodization | Metal oxide [70,71,72,73] | 23 nm [73] | Cylindrical/hexagonal prism | Array (pore density of 5 × 1010 cm−2) [73] | Good controllability when fabricating >20 nanopores | Low (requires a voltage source and etchant solutions) | Etching rate of 40 μm/h at 30 °C under bias voltage of 70 V [73] |
| Ion-track etching | Polymers [67,68,69,82,83,115] | 51 nm (in an array) [82], 2 nm (individual pores) [83] | Cylindrical | Array (pore density of 107 to 109 cm−2) [82] | Poor distribution in the pore size (standard deviation of the pore size was 22% and 25% [82]) | Middle (requires heavy ion accelerometers) | UV radiation for 10 to 24 h, and NaOH etching for 5 min on the 23 µm-thick PET [82] |
| Feedbackcontrolled wet etching | Si [65,66] | 30 nm [66] | Truncated-pyramidal | Array (pore density of 1.96 × 106 cm−2) [66,78] | Poor size uniformity for ~30 nm nanopores [66]. Good size uniformity (± 5%) for >500 nm nanopores [78]. | Low (requires heated KOH etchant) | 84 µm/h in 33 wt % KOH at 80 °C |
| EBL-assisted RIE | SiN [85], SiO2 [86,116] | 18 nm [86] | Cylindrical | Array (pore density of 5 × 1010 cm−2) [86] | Good size uniformity of 18 ± 2 nm. | High (requires EBL technique) | Hours to form patterns at the wafer-scale via EBL |
| EBL-assisted nanoimprint | Polymer [88], Al2O3 [89] | 10 nm [88] | Cylindrical/hexagonal prism | Array (pore density of 2.6 × 1011 cm−2) [88] | Good controllability thanks to the high-precision EBL | High (EBL is more expensive than the photolithography technique) | Hours to form high-precision masks at the wafer-scale via EBL |
| Metal deposition and heating | SiN [91,92], SiO2 [91] | 8 nm [90] | Conical | Array (4 × 106 cm−2) [90] | Poor distribution in the pore size and location | Middle (requires metal nanoparticles and a furnace) | Several hours for heating Au at 1067 ± 5 °C [90] |
| Shrinking by FIB/FEB | SiN [52,96], Metal oxide [97], Si [98,99], SiO2 [100] | <1 nm | Cylindrical/conical | One or several pores at a time | Sub-nanometer precision | High (requires FIB, SEM or TEM) | Shrinking rate of 0.67 nm/s [98] |
| Shrinking by material deposition | Al2O3 [103], SiN [106] | <1 nm | Cylindrical/conical | Wafer scale | Good (ALD has sub-nanometer precision [103]) | Middle (depends on the deposition technology) | Shrinking rate of 0.1 nm/cycle by ALD (1 cycle takes several seconds) |
| Shrinking by thermal oxidation | Si [107,108] | <1 nm | Cylindrical/conical | Wafer scale | Good | Middle (requires an oxidation furnace) | Shrinking rate of 4.6 nm/h [108] |