Nanofabrication & Lithography

Patterning the world at the nanoscale. Learn the cleanroom environment, photolithography (resists, masks, exposure, development), the Rayleigh resolution criterion and depth of focus, resolution enhancement (immersion, phase-shift masks, multiple patterning, OPC), extreme-ultraviolet lithography, electron-beam lithography and write time, nanoimprint, focused ion beams, thin-film deposition (PVD, CVD, ALD), wet and dry etching (RIE, anisotropy), lift-off, self-assembly and directed self-assembly, and MEMS/NEMS — with Python for resolution, depth of focus and e-beam write time.

Nanofabrication creates precise structures — transistors, sensors, photonic devices — by repeatedly depositing, patterning and etching materials. Semiconductor fabs pattern billions of features only a few nanometres wide on every chip.

Lithography is the heart of it: its resolution limit has driven the industry from 1 µm to today's EUV-patterned '3 nm class' processes.

An analogy

Photolithography is like screen printing with light: a stencil (mask) blocks light, a light-sensitive coating (resist) records the pattern, and chemicals etch away what isn't protected — repeated dozens of times with perfect alignment.

The patterning cycle

Each layer repeats:

Pushing resolution

Resolution enhancement techniques: immersion lithography (water, NA 1.35), off-axis illumination, phase-shift masks, optical proximity correction (computationally distorting mask shapes), and multiple patterning (litho-etch-litho-etch, self-aligned double/quadruple patterning) to halve or quarter pitch. EUV (13.5 nm, reflective optics in vacuum, tin-plasma sources) simplifies advanced nodes; High-NA EUV (0.55) is next.

Deposition and etching

PVD (sputtering, evaporation), CVD (LPCVD, PECVD), ALD (angstrom-level control of high-k gates and conformal liners), epitaxy (MBE, MOCVD). Wet etching is isotropic and selective; dry reactive-ion etching (RIE, DRIE Bosch process) is anisotropic for vertical walls and high aspect ratios. Atomic layer etching gives ultimate control. CMP planarises between layers.

Alternative nanopatterning

Nanoimprint lithography stamps patterns into resist (low cost, used for photonics and storage); focused ion beams mill and deposit directly (TEM sample prep, circuit edit); directed self-assembly of block copolymers produces sub-10 nm lines; dip-pen and scanning-probe lithography write molecules. MEMS/NEMS combine these with release etches to build accelerometers, gyros, microphones and nanomechanical resonators.

Formulas

Worked example: Resolution

Given: λ 193 nm, NA 1.35, k₁ 0.28

Find: R.

Answer: 40 nm half-pitch

Worked example: Double patterning

Given: Single-exposure pitch 80 nm

Find: Pitch after SADP.

Answer: 40 nm

Worked example: Write time

Given: 10 mm² at 20% fill, 500 µC/cm², 5 nA

Find: t.

Answer: 2000 s ≈ 33 min

Lithography resolution and e-beam write time

# Lithography resolution (Rayleigh), depth of focus and e-beam write time
for name, lam, NA, k1 in (("i-line", 365, 0.6, 0.6), ("ArF dry", 193, 0.93, 0.4), ("ArF immersion", 193, 1.35, 0.3), ("EUV", 13.5, 0.33, 0.4)):
    R = k1 * lam / NA
    DOF = 0.5 * lam / NA**2
    print(f"{name:14s}: resolution {R:6.1f} nm, depth of focus {DOF:6.1f} nm")
# E-beam lithography: time to write 1 cm2 at 50% fill, dose 300 uC/cm2, beam current 2 nA
area_cm2, fill, dose, I = 1.0, 0.5, 300e-6, 2e-9
t = area_cm2 * fill * dose / I
print(f"e-beam write time {t/3600:.1f} h -> why EBL is for masks and prototypes")
# i-line        : resolution  365.0 nm, depth of focus  506.9 nm
# ArF dry       : resolution   83.0 nm, depth of focus  111.6 nm
# ArF immersion : resolution   42.9 nm, depth of focus   52.9 nm
# EUV           : resolution   16.4 nm, depth of focus   62.0 nm
# e-beam write time 20.8 h -> why EBL is for masks and prototypes

Resolution improved by shortening wavelength (365 → 193 → 13.5 nm), raising NA (immersion in water gives NA > 1) and lowering k₁ with clever tricks. The price is shrinking depth of focus, demanding extremely flat wafers. E-beam lithography can write any pattern at < 10 nm but takes ~21 hours per cm² — ideal for masks and research, not mass production.

Common mistakes

Applications

Key takeaways

Going further