Photolithography (EUV)

Inside the source vessel, molten tin droplets about 25 micrometers across fall at 50,000 per second. A high-power CO2 laser fires a pre-pulse to flatten each droplet into a pancake, then a 25 kW main pulse vaporizes it into plasma at roughly 220,000 K. The plasma emits a narrow band of light, including the 13.5 nm EUV line. A collector mirror focuses this light into the scanner. Conversion efficiency is only around 5%, so generating useful EUV power requires hundreds of kilowatts of CO2 laser input.

At 13.5 nm, every known transparent material absorbs the light, including fused silica, quartz, and CaF2 used at 193 nm DUV. Glass simply will not transmit EUV. Instead the optical path uses six to ten reflective mirrors coated with 40 to 50 alternating bilayers of molybdenum and silicon, each layer tuned to a quarter wavelength so reflections add coherently. Even with this Bragg stack each mirror reflects only about 70%, so a six-mirror system delivers around 12% of source light to the wafer. The whole optical path runs in vacuum because EUV is also absorbed by air.

Before EUV, foundries pushed 193 nm immersion DUV below its diffraction limit by splitting one mask into two, three, or four exposures, each printing a subset of features. This is double, triple, or quadruple patterning. Each extra exposure adds a full lithography cycle: coat, expose, develop, etch, plus overlay metrology. A single dense metal layer at 7 nm could need four DUV exposures versus one EUV exposure. EUV's shorter wavelength and higher numerical aperture (0.33 today, 0.55 with High-NA) prints features in a single shot, halving cycle time and reducing alignment-error stack-up.

The minimum metal pitch on TSMC N3E is around 23 nm, gate pitch around 45 nm, and a single 6T SRAM cell occupies 0.0199 square micrometers. A FinFET fin is roughly 6 nm wide. Density on a high-density library reaches roughly 292 million transistors per square millimeter. Compared to a 28 nm node from 2011 (approximately 5 million transistors per square millimeter) that is a 50-plus times shrink. To picture it, a transistor gate is now smaller than the SARS-CoV-2 virion (about 100 nm), and the fin is comparable to a strand of DNA in width.

Numerical aperture (NA) is the sine of the half-angle of light a lens or mirror system can collect. Standard EUV scanners have NA = 0.33; High-NA EUV (the EXE:5000 series) raises this to 0.55. Higher NA means smaller resolvable features (resolution roughly k1 times wavelength divided by NA). The mirrors are larger and anamorphic — magnification is 4x in one axis but 8x in the other to keep the mask manageable — so each exposure field is half-height (16.5 mm rather than 33 mm). Throughput drops, but a single High-NA exposure replaces what would need EUV double-patterning at the 2 nm node.

EUV masks are reflective multilayer Mo/Si stacks with a patterned absorber on top. A single particle a few tens of nanometers across can ruin every wafer printed afterward, and one mask costs $300,000 to $500,000. Protection uses a pellicle, a freestanding membrane held a few millimeters above the mask, so airborne particles land on the pellicle (out of focus) instead of on the absorber. EUV pellicles are extraordinarily hard to make: they must transmit better than 90% at 13.5 nm and survive several kilowatts per square centimeter of EUV flux without sagging or burning. Modern pellicles are roughly 30 nm thick polysilicon membranes.