Wafer steppers - and wafer scanners - are the most important manufacturing machines required in chip fabrication. In a way similar to an oversized slide projector, the structures of the subsequent semiconductor components are transferred in a wafer stepper from a mask (reticle) to a wafer coated with photoresist. This stage of production - termed photolithography - is the most important manufacturing process in the fabrication of semiconductor components.
The centerpiece of a wafer stepper is the optical systems for illumination and projection. StarlithŪ optical systems from Carl Zeiss SMT AG are the worldwide leading stepper and scanner optics. They are utilized in PAS 5500 and Twinscan plattforms from SMTīs partner ASML BV (Veldhoven, NL).
The illumination optics (comparable to the condenser in a slide projector) ensures the even illumination of the mask (corresponds to the slide). The lithography lens projects the mask structures on to the wafer (unlike a slide projector, however, the structures are demagnified, typically by a factor of four). After each exposure process, the wafer is moved under the optical system by exactly the size of the image field covered and is thus completely exposed "step by step" - hence the name wafer stepper.
For even better process parameters socalled scanners, which continuously scan the mask (in steppers the mask is exposed at one go) are utilized for several years now.
Illumination wavelength
The higher the performance of a chip is to be, the finer the structures which the optical system must be able to produce. For this reason, advances in miniaturization and thus the constantly increasing performance of integrated circuits are directly dependent on the resolving power provided by the optical system used. Just like a painter, who needs ever finer paint-brushs to get ever finer details onto the canvas, for optical lithography ever shorter wavelengths are used to shrink feature sizes. Carl Zeiss SMT offers for all wavelengths used in chipfabrication at present leading edge optical systems. These range from 365nm (g-line) to 248nm (KrF), 193nm (ArF), 193 immersion to 13,5 nm (EUVL).
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