Extreme Ultraviolet Lithography

Extreme ultraviolet lithography, EUV, is a photolithography technique that uses light of about 13.5 nanometres to print extremely small circuit patterns on semiconductor wafers. Its short wavelength improves optical resolution, allowing critical transistor and interconnect layers to be patterned more densely than with 193 nm deep-ultraviolet lithography.

Principle of finer patterning

Lithographic resolution depends mainly on the wavelength of light, the optical system's numerical aperture and process control. A shorter wavelength permits smaller features and tighter spacing, although the printed dimensions also depend on the mask, optics, photoresist and computational correction techniques.

• EUV can reduce the need for repeated multiple patterning that 193 nm systems require to create some dense features.
• Fewer patterning steps can reduce process complexity and opportunities for alignment errors on critical layers.

How the pattern reaches the wafer

• High-energy laser pulses strike tin droplets to produce a plasma that emits 13.5 nm EUV radiation.
• Because EUV is strongly absorbed by ordinary materials and air, the system operates in a vacuum and uses reflective rather than transmissive optics.
• Multilayer mirrors collect and direct the radiation onto a reflective mask carrying the circuit pattern.
• Projection mirrors reduce and focus the mask pattern onto a light-sensitive photoresist coating on the silicon wafer.
• The exposed resist is developed, and the pattern is transferred into underlying material through etching or deposition; the cycle is repeated for successive chip layers.

Why EUV is technologically demanding

EUV enables advanced chips only through exceptionally precise control of light generation, masks, mirrors, wafer positioning and materials. Mirror contamination, mask defects, limited usable source power and random photoresist effects can reduce pattern fidelity, throughput and manufacturing yield.

• EUV complements rather than replaces deposition, etching, metrology and other semiconductor-fabrication processes.
• Its specialised equipment and process knowledge make EUV capability a major technological barrier in leading-edge chip manufacturing.

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