The Next Frontier of Microfabrication: Exploring Advanced Lithography Methods

It was Moore’s Law that led Kodak to predict that consumer digital photography would one day be widely available. As a result, Kodak was able to get ahead of the competition by preparing for the advent of digital photography in 1973. Today, Gordon Moore’s theory that the number of transistors on a microchip would double every two years is facing a slowdown. As demand for modern chips continues to rise unabated, semiconductor lithography, including EUV lithography, is being pushed to its absolute limits. 

The Role of Lithography in the Digital Age

Lithography, meaning ‘writing on stone’ in Greek, refers to a traditional printing method utilising oil-based inks, water, and gum arabic. Today, discussions involving lithography are more likely than not referring to photolithography, the process for patterning integrated circuits, or microchips, which uses light to transfer a design pattern from a mask onto a light-sensitive material coated on a silicon substrate.

This allows for the creation of billions of microscopic transistors. The relentless drive of Moore’s Law, which requires doubling transistor density, has resulted in the development of advanced lithography methods, such as Deep Ultraviolet (DUV) and Extreme Ultraviolet (EUV). This evolution of lithography from using visible light down to shorter and shorter wavelengths is necessary to achieve the required sub-nanometer resolution.

The Limits of Traditional Lithography

Traditional photolithography is constrained by the diffraction limit, which dictates that the minimum printable feature size is proportional to the light’s wavelength (L∝λ).

Older Deep Ultraviolet (DUV) lithography, using 193 nm light, eventually reached its physical limit for mass production of advanced chips. Techniques such as immersion lithography, which uses a liquid between the lens and the wafer, and complex multi-patterning methods emerged as interim solutions. Such lithography methods pushed DUV to its absolute limit before the transition to next-generation techniques became essential.

Extreme Ultraviolet (EUV) Lithography: A Game Changer

Extreme Ultraviolet (EUV) lithography is the current leading-edge technique for high-volume semiconductor manufacturing. It uses a complex laser-produced plasma to generate extremely short-wavelength light down to 13.5 nm. This is in contrast to the highest resolution DUV systems that generate 193 nm light. 

DUV vs. EUV Lithography Methods

Feature	DUV Lithography	EUV Lithography
Wavelength	193 nm	13.5 nm
Optics	Transmissive; uses lenses to focus light	Reflective; uses mirrors to reflect light
Environment	Dry or immersion system	High vacuum environment to prevent light absorption by air

Since virtually no material is transparent at this wavelength, EUV systems face unique engineering challenges, as they rely on a high vacuum environment and require sophisticated reflective optics instead of traditional lenses to direct light. 

Today’s advanced chips are manufactured using a combination of DUV and EUV lithography. In just decades, humans have gone from fitting just 1,000 transistors in a microchip in 1970, to fitting over 57 billion components in an area slightly larger than a fingertip.

Challenges and Innovations in EUV

EUV lithography uses complex photonics solutions involving liquid tin, specialised mirrors and photomasks to create desired wavelengths of 13.5 nm. Powerful lasers vaporise liquid tin droplets in a vacuum chamber, resulting in plasma that emits EUV radiation, which is then collected and directed by Bragg reflectors, multi-layer mirrors coated with atomic-thin layers of molybdenum (Mo) and silicon (Si). 

Unlike transparent DUV photomasks, EUV photomask reticles are reflective. When EUV light hits the mask, areas without the absorber pattern reflect the light to the wafer, while areas with the absorber pattern absorb it, creating the chip’s blueprint. Photoresist materials highly sensitive to EUV photons are needed to maximise throughput, while simultaneously maintaining the extreme resolution and pattern fidelity demanded by modern chip designs.

Two-Photon Lithography (2PL): Enabling 3D Nanofabrication

A versatile direct laser writing technique used in research and specialised fields, Two-Photon Lithography (2PL) is capable of creating 3D objects with nanoscale precision. The underlying principle uses a high-intensity, femtosecond laser to trigger a two-photon absorption process within a specialised photoresist. 

Unlike conventional lithography, polymerisation only occurs precisely at the focal point where the light intensity is highest, allowing the laser to be moved through the volume of the material to write intricate, high-resolution 3D objects. A key advantage of 2PL is its ability to create complex, overhang-free structures without relying on multiple masks or a slow layer-by-layer stacking process. Perfect for custom lithography projects, 2PL lithography is also essential for photonics applications, nano-robotics and the creation of specific biomedical devices.

Applications of 2PL Beyond Electronics

Two-Photon Lithography (2PL) is used to produce intricate lenses, waveguides, and diffractive optical elements for compact sensors and communication. For biomedical engineering, 2PL enables the creation of complex, biocompatible scaffolds for tissue engineering and high-precision micro-needles for painless drug delivery. 2PL is also a key tool for developing artificially engineered composite materials, also known as metamaterials, for optical or mechanical properties not found in natural materials. For example, the Massachusetts Institute of Technology (MIT) developed a double-network polymer that is both strong and flexible. MIT researchers say this metamaterial could have many applications, from building flexible semiconductors, electronic chip packaging, to miniature scaffolds designated for growing tissue repair cells.

How NSTIC is Driving Lithography Innovation

Singapore is making strategic investments in advanced semiconductor R&D to cement its position as a global innovation hub. In the past two years, the city-state has secured over S$18 billion in semiconductor investments. The National Semiconductor Translation and Innovation Centre (NSTIC) is a key initiative in the effort, serving as a crucial platform for both industry and research.

NSTIC directly drives advanced lithography innovation by bridging the R&D gap. It provides startups, SMEs, and researchers with access to state-of-the-art lithography tools that would otherwise be inaccessible to all but the most advanced labs, rapidly accelerating the translation of pioneering research into commercial-ready prototypes.

The Centre’s focus on silicon photonics and advanced packaging allow for the manufacturing of high-performance devices for applications in AI, quantum computing, and high-speed communications through cutting-edge, high-precision lithography. In line with the government’s push to develop world-leading capabilities in advanced packaging, heterogeneous integration and photonics, NSTIC is dedicated to nurturing the next generation deep-tech talent. Through collaborations with Institutes of Higher Learning, we aim to secure a strong pipeline of skilled lithography and fabrication professionals.

Conclusion: Pushing the Boundaries of Innovation