Other than their use of different wavelengths, EUV and DUV scanners are similar instruments. Since there are no substantial transparent materials for EUV light, the illumination and projection optics must be catoptric and the light path must be in a vacuum. Much more effort and a greater cost are needed to switch the wavelength from 193 to 13.5 nm than to switch it from 248 to 193 nm.
In a DUV scanner, light with a 193-nm wavelength emitted from an argon-fluorine (ArF) excimer laser is directed to a reticle by illumination optics that consist of many optical elements. Images on a reticle are projected onto a wafer by projection optics of 1â4 magnification. A reticle and a wafer are each mounted on the scanning stage, and they are run synchronously at a speed that corresponds to the magnification of the projection optics. Fiducial marks on the stages are used for various calibrations. A wafer alignment sensor detects the position of the wafer alignment mark on a wafer. The distance between the wafer surface and the projection optics is measured with a wafer focus sensor.
An EUV scanner uses light with a 13.5-nm wavelength from a laser-produced plasma (LPP) source or a discharged-produced plasma (DPP) source. The illumination and projection optics consist of reflective optical elements; EUV reticles are a reflective type as well. The reflective elements are coated with molybdenum-silicon (MoâSi) multilayers (MLs), and the reflectivity is in the range of 60 to 70%. Because the reflectivity of MoâSi-coated elements can be easily degraded by carbon contamination or oxidation, the partial pressure of hydrocarbon and water inside EUV scanners must be controlled to a very low level. However, the body, stage, wafer alignment sensor, and wafer focus sensor can be designed with optical lithography technology.