What Is Lithography?

Lithography is a patterning technique in which a geometric design is transferred from a mask or template onto a substrate material, most commonly a semiconductor wafer coated with a light-sensitive polymer called a photoresist. In semiconductor manufacturing, lithography defines the dimensions and positions of transistors, interconnects, and other features that make up integrated circuits. The minimum feature size that lithography can reliably resolve sets the technology node of a manufacturing process, and advancing that resolution limit has been the central engineering challenge of the semiconductor industry for more than fifty years.

The word derives from the Greek for "stone writing" and refers historically to an offset printing technique, but in modern engineering usage it almost exclusively denotes optical or charged-particle pattern transfer processes used to fabricate electronic and photonic devices. IEEE's International Roadmap for Devices and Systems (IRDS) tracks lithography capability requirements and projected technology transitions across process generations, with the IRDS 2022 Lithography chapter providing the current roadmap for feature scaling.

Photolithography

Photolithography uses light passed through a patterned mask to expose a photoresist layer. The exposed regions either harden (negative resist) or soften (positive resist), allowing selective etching or deposition of materials to build up device structures. For many decades, mercury arc lamps at 365 nm (i-line) and then 248 nm (krypton fluoride) and 193 nm (argon fluoride) excimer lasers served as the illumination sources. At the 193 nm wavelength, immersion lithography, which replaces the air gap between lens and wafer with deionized water to increase the effective numerical aperture, allowed the industry to extend the technology to sub-45 nm nodes without a change of light source. Proximity effects, in which diffracted light from one feature bleeds into neighboring features, are corrected computationally using optical proximity correction (OPC) algorithms applied to mask data before fabrication.

Extreme Ultraviolet Lithography

Extreme ultraviolet (EUV) lithography uses 13.5 nm light generated by a laser-produced tin plasma, reducing the wavelength by more than an order of magnitude compared with argon fluoride immersion tools. This shorter wavelength enables patterning of features at the 5 nm and 3 nm process nodes with fewer multiple-patterning steps, substantially reducing manufacturing cost and cycle time. As reported in IEEE Spectrum, ASML's NXE series EUV scanners entered high-volume manufacturing at Samsung, TSMC, and Intel beginning around 2018 to 2019. High-NA EUV tools with numerical aperture of 0.55, compared with 0.33 for first-generation EUV scanners, are targeting sub-2 nm patterning capability.

EUV introduces its own proximity effects through mask 3D shadowing and stochastic variations in the small number of photons that expose each feature at current dose levels, driving research into better photoresists and computational mask correction methods.

Alternative Patterning Techniques

Electron beam lithography (EBL) uses a focused electron beam to write patterns directly on resist without a mask, enabling sub-10 nm feature definition at the cost of low throughput. EBL is used routinely for mask making, research prototyping, and photonic device fabrication. Nanoimprint lithography (NIL) mechanically stamps a rigid template into a soft resist layer, transferring patterns with nanometer resolution at high speed. Ion beam lithography uses focused gallium or helium ion beams for both patterning and circuit editing. Nanoimprint tools from Canon have been positioned as a potential complement to EUV at leading-edge nodes, offering a different cost and throughput trade-off profile.

Applications

Lithography has applications across a wide range of microfabrication and nanofabrication domains, including:

• Integrated circuit fabrication at leading-edge semiconductor foundries
• MEMS sensor and actuator fabrication
• Photonic integrated circuit and waveguide patterning
• Flat panel display manufacturing
• Microfluidic chip and lab-on-a-chip device fabrication