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