LITHOGRAPHY PRINCIPLES

Lenses & mirrors

The complex optical systems in our machines reduce the image printed on a wafer to the nanometer level

When we talk about a lithography machine’s lens, we aren’t talking about a single, simple lens. We’re talking about a complex optical system made up of dozens of individual lenses, and, for EUV lithography, mirrors.

Each one must be correctly positioned to within a nanometer to ensure image quality. ASML’s innovation in lens design allows chipmakers to reduce the size of features on a microchip. Since the late 1980s, all our lithography systems have featured optics from our strategic partner ZEISS.

Numerical aperture

Lens development to improve resolution means increasing the numerical aperture (NA), a measure of how much light the lens system can collect and focus. One way to do that is by integrating higher-precision lenses and mirrors into extended optical systems. The highest NA optical systems today are over 1.2 meters high and weigh more than a metric ton. Used in DUV lithography, they have an NA of 1.35.


Immersion lithography

In 2003, ASML made an important step forward in numerical aperture. We developed immersion lithography, which allows chipmakers to print even smaller features by projecting light through a layer of water between the lens and the wafer. The water increases the numerical aperture of the systems' optics.

Complexity brings control

The sheer complexity of today’s lithography optics makes delivering high-quality optical systems a massive engineering challenge. But it also brings a unique opportunity to control every exposure with immense precision. Thousands of actuators allow the exact position and orientation of individual lens and mirror elements to be minutely adjusted to ensure the perfect image on the wafer every time.


For example, the repeated intense light pulses used in lithography cause the optical system to warm up, which leads to distortions in the lens. These changes may be measured in millikelvins and nanometers, but that’s still enough to lead to defective microchips. Actuators in the optical elements can actively compensate for these lens heating effects.

EUV needs mirrors

Other lithography machines use lenses to focus light. But there are no lenses for extreme ultraviolet (EUV) lithography. Since most materials absorb EUV light, the lenses would absorb the light in the system. Instead, we developed a brand-new optical system that uses ultra-smooth, multilayer mirrors inside a vacuum chamber. Each mirror has over 100 layers of materials that are carefully chosen and precisely engineered to maximize the reflection of EUV light.


Flatness is crucial. The mirrors are polished to a smoothness of less than one atom’s thickness. To put that in perspective, if the mirrors were the size of Germany, the tallest ‘mountain’ would be just 1 mm high.

High-NA EUV

ASML is developing a next-generation EUV platform that increases the numerical aperture from 0.33 NA to 0.55 (‘High-NA’). This platform has a novel optics design and significantly faster stages. It will enable geometric chip scaling beyond the next decade, offering a resolution and overlay capability that is 70% better than our current EUV platform. The High-NA platform has been designed to enable multiple future nodes, starting at the 3 nm Logic node and followed by Memory nodes at similar density.

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