What is DUV lithography?

The production of microchips requires the most advanced technologies to create the finest structures on thin silicon wafers. These structures form an integrated circuit (IC), i.e. a microchip. DUV lithography optics ( d eep u ltra v iolet light) from ZEISS Semiconductor Manufacturing Technology (SMT) use light in the invisible ultraviolet spectrum with wavelengths of 365, 248 and 193 nanometers for precise exposure. Over the past 50 years, ZEISS SMT has reduced the wavelength of light even further and made progress in resolution to accommodate smaller structures and more powerful chips. As technology leader, we offer state-of-the-art systems for dry lithography (Dry DUV) and immersion lithography (DUV Immersion) and are actively shaping the future with new product developments.

How does a DUV lithography system work?

Light source, illumination system, mask, projection optics and wafer are the central components of a DUV lithography system: mercury vapor lamps or specialized gas lasers, so-called excimer lasers, generate DUV light in the ultraviolet wavelength range. ZEISS SMT provides the optics for the excimer lasers through its Optics Modules division. These optics must withstand high light intensities and low wavelengths. The information – a type of blueprint for the microchip – is located on a projection mask (reticle). The optical system from ZEISS SMT uses DUV light to project the pattern onto silicon wafers reduced by a factor of four. This makes the DUV lithography system comparable to an "inverted slide projector”.

What is DUV lithography used for?

Whether it´s in cars, smartphones, smart refrigerators, or computers, almost all devices in our digitized lives and working environment contain microchips. These chips are simply not possible without DUV technology. Since the technology is both economical and powerful, chip manufacturers continue to use various DUV wavelengths – 193, 248 and 365 nanometers – to produce most layers of microchips. Even for microchips made with EUV technology , the majority of IC layers (integrated circuits) are based on DUV technology. Around 80 percent of all microchips worldwide are manufactured using ZEISS optics and lithography machines from our strategic partner ASML . The majority of these – more than 95 percent – are produced with DUV light. This makes ZEISS the market leader and setting the pace for the semiconductor industry.

Why will DUV lithography continue to play an important role in the future?

The demand for microchips continues to rise rapidly due to increasing digitization and future trends such as the Internet of Things, artificial intelligence and smart cities. Microchips must be robust in application and cost-effective to produce. This is possible thanks to the mature and widely established DUV technology. It covers the market needs, in particular for basic chips. Advanced chips combine EUV and DUV lithography in the manufacturing process. Today and in the future, most layers of a high-end microchip are produced using DUV light from the "dry" systems (DUV dry). As enablers of digitalization DUV, EUV, and High-NA EUV lithography will continue to exist side by side as complementary technologies.

Two employees of ZEISS SMT works on a DUV technology

DUV lithography

DUV lithography optics from ZEISS

Resolution and precision drivers of innovation

Shorter, finer, more precise – and invisible to humans

The spectrum of light visible to humans is approximately between 400 and 800 nanometers in wavelength. This is far too long for today's demands in semiconductor manufacturing. Exposing the fine structures of microchips onto silicon wafers requires wavelengths below the spectrum visible to humans. With lithography optics from ZEISS SMT (no sales in Germany), chip manufacturers worldwide can expose with nanometer precision – in the range of "deep ultraviolet light" (DUV light) with wavelengths of 365, 248 and 193 nanometers.

How DUV lithography works

The DUV light required is generated by excimer lasers. These are gas lasers that can generate electromagnetic radiation in the ultraviolet wavelength range. Excimer lasers are currently the most flexible and powerful sources of light in the ultraviolet region of the spectrum.

The picture shows a module that is installed in excimer lasers.
The optics for the excimer lasers must operate at high light intensities and low wavelengths. The Optics Modules (OM) segment of ZEISS SMT supplies these optics.
Depending on the wavelength, different gases are used. At 248 nanometers it is krypton fluoride (KrF), at 193 nanometers the light wavelength is generated with argon fluoride (ArF).
The projection mask, or reticle, contains the blueprint for a microchip.
Using the optical system from ZEISS SMT, the information on the mask is reduced in size and projected onto silicon wafers. Similar to a slide projector in which the image is projected onto a screen, the DUV light reproduces the pattern of the mask on the wafer at a greatly reduced size.
The quality and form of the illumination system and the resolving power of the projection optics from ZEISS also determine how small the structures on a microchip can be.
With DUV light, structures down to a resolution of 40 nanometers are possible – thanks to a new process used for the first time in DUV lithography: the immersion lithograph

Numerical aperture

Better resolution through immersion

Resolution in previous lithography technologies was limited by the air space above the wafer. Ernst Abbe already formulated that the resolution of light microscopes is limited by the wavelength of the light and the numerical aperture (also called Abbe limit). The numerical aperture results from the refractive index of the last medium above the image plane and the aperture angle of the optics. The aperture angle of the optics in turn depends on the size of the optics. Older lithography technologies have reached an economically viable limit here. A new approach is needed if the resolution is to be improved. The solution lies in an immersion liquid that fills the air space above the wafer. Abbe had already researched the immersion principle in microscopy and was now able to use it successfully with immersion optics in DUV lithography.

Employee screws on a DUV product Starlith 1900i

Using immersion for an improved resolution

In microscopy, the method has long been proven. Since the mid-2000s, it has also been used within the optics for microchip manufacturing at ZEISS SMT. A liquid is introduced between the optics and the wafer and the optical head is dipped in it (immersion). Due to the higher refractive index of the water, the light beam is deflected more strongly, which increases the numerical aperture, and the resolution improves decisively. For example, ZEISS lithography optics with the light wavelength of 193 nanometers can achieve resolutions of less than 40 nanometers.

Highly flexible illumination systems for imaging optimization

Pushing the limits of optical resolution, the choice of the illumination setting plays a crucial role in optimizing the imaging process. Illumination setting and mask layout are co-optimized to ensure that the imaging process prints on target and has sufficient tolerance against process variations. To support most advanced Source-Mask-Optimization, ZEISS illumination systems provide virtually infinite degrees of freedom for customer-specific optimization. Since 2009, the immersion systems are equipped with the FlexRay illuminator: an array of micro-mirrors allows to realize user-defined illumination settings in real-time, without any lead times, ensuring highest and most robust imaging quality for even the most advanced chip designs.

ZEISS as a technology leader

Our strategic partner ASML – with optics from ZEISS SMT – was the first manufacturer worldwide to bring immersion lithography to production maturity. With this prototype of immersion optics, ZEISS SMT changed the optical lithography roadmap in 2003. Whereas 157-nanometer lithography was previously considered the technology of the future, immersion lithography has now become established as a way of continuing Moore's Law. Today, ZEISS lithography optics in wafer steppers and wafer scanners from our strategic partner ASML are core elements of modern microchip production and set the pace for the semiconductor industry.

To Optical Lithography

Around 80 percent of all microchips worldwide are manufactured using optics from ZEISS.

Dr. Thomas Stammler Chief Technology Officer

DUV technology highlights