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

DUV lithography optics from ZEISS SMT: No sales in Germany

Lithography at 193 nanometers (ArF)

An increase in resolution is achieved with immersion technology. The Starlith® 1982i from ZEISS is one of the company's most successful and best-selling products. It enables resolutions below 40 nanometers using an excimer laser with argon fluoride (ArF).

The Starlith® 1460 from ZEISS is a lithography optic that enables resolutions of 55 nm. It is used worldwide in volume production of microchips and works with the design as a "dry" system, i.e. there is air between the last lens and wafer.
Lithography at 248 nanometers (KrF)

The Starlith® 860 lithography optics is one of the best-selling ZEISS SMT optics for excimer lasers with krypton fluoride (KrF). It enables resolutions from 110 to 90 nanometers.

The Starlith® 1000 is likewise a volume product. It works with the wavelength of 248 nanometers and achieves resolution of down to 80 nanometers.
Lithography at 365 nanometers (I-Line)

The Starlith® 400 from ZEISS operates at a wavelength of 365 nanometers and is used, for example, in the lithography of non-critical structures. The optics enable structures of 220 nanometers and use a high-pressure mercury vapor lamp.
Line Narrowing Module (LNM)

The radiation generated in the laser chamber is quasi-monochromatic and yet must be further reduced in wavelength bandwidth to avoid imaging errors when projecting the structures from the photomask onto the wafer.

As a bandwidth reduction module, the Line Narrowing Module (LNM) performs this task by splitting the laser light and reducing it to the desired bandwidth of the wavelength.
Optical components for lasers

The deep ultraviolet radiation of DUV lasers has a very high energy density, which can lead to degradation phenomena in materials used in the laser. ZEISS SMT has therefore specialized in understanding the physics and chemistry of degradation in optical materials and developing resistant optical components.

These can perform various tasks in the laser: Prisms that widen or reduce the beam, beam splitters that reflect part of the light and allow the other part to pass, and reflective and antireflective coatings.
Illuminaton Systems

The illumination of the reticle plays a crucial role in optimizing the lithographic imaging process – flexibility and control are key. For each type of projection optics a specific illumination system is developed. This makes sure that the modules work together for optimum performance at our customers’ sites.