Deep Ultraviolet (DUV) and Extreme Ultraviolet (EUV) lithography create semiconductor features using fundamentally different optical approaches. DUV projects light through lenses, while EUV relies entirely on mirrors.
The reason lies in the behavior of light and materials at extremely short wavelengths.
Why wavelength determines everything in lithography
The fundamental goal of lithography is to project light through a patterned mask and expose a photosensitive coating on a silicon wafer. The smallest feature that can be printed is approximated by the Rayleigh equation (k₁ × λ / NA), showing that smaller features require shorter wavelengths, larger numerical apertures, or process improvements.
The history of optical lithography has largely been driven by the pursuit of shorter wavelengths. Mercury arc lamps gave way to excimer lasers as the industry moved from g-line (436 nm) to i-line (365 nm), then to krypton fluoride (KrF) at 248 nm and argon fluoride (ArF) at 193 nm. Each reduction in wavelength enabled smaller feature sizes, supporting successive generations of denser and more capable semiconductor devices.
Wavelength reduction does not happen in isolation. Every advance depends on optical materials that can transmit, shape, and control the light effectively. As wavelengths get shorter, the number of materials capable of doing that becomes increasingly small.
How DUV lithography works and the role of optical components
DUV lithography, specifically ArF at 193 nm, has been the semiconductor industry’s primary patterning technology since the early 2000s. At this wavelength, light from an ArF excimer laser passes through an illumination system, through a transmissive reticle containing the circuit pattern, and through a multi-element projection lens system before exposing the photoresist on the wafer.
The key word is through. At 193 nm, fused silica (SiO₂) and calcium fluoride (CaF₂) transmit light efficiently, making them suitable for projection lenses, condenser optics, beam delivery systems, and transmissive reticles.
The water trick that extended DUV’s life
By the mid-2000s, engineers were approaching the resolution limit of 193 nm DUV. Immersion lithography extended its capability by filling the gap between the final lens element and the wafer with ultra-pure water, reducing the effective wavelength in the imaging medium to around 134 nm.
Combined with multiple patterning, immersion DUV has been used in manufacturing processes associated with 5-7 nm process nodes at leading-edge fabs.
Optical components in DUV systems
For suppliers of precision optical components, DUV systems represent a significant and enduring market encompassing projection optics, reticles, beam delivery components, and inspection systems. Precision reticles remain a critical part of the DUV optical chain, serving as the pattern source that ultimately defines the structures printed onto the wafer.
Precision reticles remain a critical part of the DUV optical chain, serving as the pattern source that ultimately defines the structures printed onto the wafer.
The limitations of DUV
If shorter wavelengths consistently unlock smaller features, why not keep going below 193 nm? In principle, the resolution equation supports it. In practice, materials physics said no.
Below approximately 150 nm, optical materials begin to absorb rather than transmit light. Fused silica, one of the most UV-transparent materials available, becomes largely opaque below 150 nm. Calcium fluoride holds out slightly longer, but not enough to be practical for chip manufacturing at scale.
F₂ lasers operating at 157 nm were investigated as a successor to ArF, but the technology proved impractical because the light is strongly absorbed by air and suitable optical materials were difficult to manufacture reliably at production scale.
The limitation became clear: transmissive optics could not support ever-shorter wavelengths indefinitely. At some point, the physics of light-matter interaction effectively closes the door on transmissive optics.
EUV operates at 13.5 nm. No material transmits 13.5 nm EUV light with any meaningful efficiency. Even air absorbs EUV strongly, which is why every EUV lithography system operates in high vacuum throughout. You cannot transmit EUV light. You can only reflect it.
How EUV lithography works and why only mirrors will work
EUV lithography uses light at 13.5 nm, technically in the soft X-ray part of the electromagnetic spectrum, though the name ‘EUV’ has stuck for historical reasons.
Because nothing transmits EUV light, the entire optical system is reflective. The projection optics, illuminator optics, and reticle are all reflective. This changes the design requirements throughout the optical system.
Multilayer mirrors: remarkable engineering under constraint
EUV mirrors use multilayer molybdenum and silicon coatings deposited on ultra-low-expansion substrates. These structures increase reflectivity at 13.5 nm, but each mirror still loses a significant proportion of the incoming light. Because an EUV system contains multiple mirrors, only a small fraction of the generated energy reaches the wafer, driving demand for extremely powerful EUV sources. The reticle is also reflective, using a multilayer mirror structure with a patterned absorber layer.
What EUV delivers
At advanced process nodes, EUV enables smaller feature dimensions than are practical with conventional DUV lithography, while reducing or eliminating the need for multiple patterning. This lowers process complexity, cycle time, and the risk of accumulated alignment error.
What EUV and DUV mean for wafer-level optics
While DUV and EUV lithography differ in how they manipulate light, both create demand for highly specialized optical structures at wafer level.
Many semiconductor devices incorporate patterned optical films, color filters, spectral filters and deposited optical coatings that must be manufactured with the same precision expected elsewhere in the semiconductor process. Image sensors, proximity sensors, biometric devices, and ambient light sensors all rely on carefully controlled optical performance integrated directly onto the wafer.
We provide wafer-level microlithography capabilities including active wafer deposition, patterned optical coatings, color filters and photoresists, and dichroic coatings for semiconductor and photonic applications.
Custom optical components in lithography applications
Both DUV and EUV applications routinely require custom optical components. Wavelength ranges are tightly specified, coating performance is non-negotiable, and surface quality and form tolerances are among the most demanding in any industry. Off-the-shelf catalog products rarely meet these requirements.
Conclusion
The transition from DUV to EUV was driven by a simple constraint: optical materials eventually stopped transmitting light efficiently at shorter wavelengths.
The industry's solution was to replace transmissive optics with reflective ones, fundamentally changing lithography system design. While DUV and EUV rely on different optical approaches, both remain essential technologies in semiconductor manufacturing.
Beyond chip patterning, the same lithographic principles are used to create wafer-level optical structures for sensing, imaging, and photonic devices, extending the role of precision optics throughout modern electronics.
From reticles and patterned coatings to integrated optical structures, precision optics remain central to semiconductor manufacturing. To discuss your application, please contact our technical sales team: sales@torrentphotonics.com