Lithography

Lithography is a crucial step in the semiconductor manufacturing process. It allows intricate patterns to be etched onto silicon wafers to create the integrated circuits that power everything from smartphones to supercomputers.

This process, known as photolithography in semiconductor production, has evolved significantly to keep up with the demands of increasingly packed microchips.

What is chip lithography?

At its core, chip lithography involves transferring a circuit design onto a silicon wafer. This is achieved by applying a light-sensitive material called photoresist to the wafer's surface. The wafer is then exposed to light through a mask that contains the desired pattern. The light alters the photoresist. After development, the pattern is etched into the wafer, forming the micro-scale features that make up the circuit.

The fundamentals of lithography were discovered by Jay Lathrop, who coined the term photolitography, derived from the greeks words for ‘drawing with light on stone’. The technology has been crucial to microchip manufacturing ever since its early days in the 1950s.

Increasingly precise: From extreme UV to High-NA EUV

As the demand for smaller, more powerful chips has grown over the past decades, so too has the need for more precise lithography techniques. Today’s lithography tools rely on the world’s flattest mirrors and most powerful commercial lasers.

Extreme UV

Extreme ultraviolet (EUV) lithography uses light with a wavelength of 13.5 nanometers, much shorter than the deep ultraviolet light previously used. This breakthrough was first deployed in chip mass production in 2019. The shorter wavelength allows for the creation of smaller, nanoscale features on the chip, enabling the production of more highly advanced and efficient semiconductors.

Producing EUV light at a large scale is one of the toughest engineering challenges ever:

• A powerful laser pulverizes a tiny 30-micron-wide ball of tin twice. This turns it into super-hot plasma that emits EUV light.
• Extremely flat mirrors made of nanometer-thick layers of silicon and molybdenum then capture this light. Ultra-precise sensors and actuators keep these mirrors nearly motionless. These sensors and actuators are so precise they could direct a laser to hit a golf ball at a distance as far as the moon.

The technology took decades and billions of dollars to develop, but the Dutch company ASML got it to work. ASML, closely collaborating with imec, is already developing the next generation of lithography technology: high numerical aperture (High-NA) EUV.

High-NA EUV

Standard EUV systems typically have a so-called numerical aperture or NA of 0.33. This number represents a key optical parameter determining the resolving power of a lithography system, meaning its ability to focus light onto a small area. Higher NAs, up to around 0.55, will allow for the creation of even smaller features on a chip.

Deploying such new technology will require the entire industry ecosystem to meet the process requirements and establish the infrastructure that goes along with high-NA tool development—think of new mask technology, measurement and screening methods, as well as the development of new materials for thin film patterning.

In early 2024, imec and ASML opened their joint High-NA EUV lab, enabling the exploratory phase of this new technology. They expect to see the introduction of the first high-NA EUV lithography equipment in high-volume manufacturing environments in 2025.

With the advent of increasingly precise lithography methods, the industry is poised to continue the trend of miniaturization and performance improvement.