Mirror substrates for EUV-lithography: progress in metrology and optical fabrication technology

EUVL, i.e. projection lithography, utilizing actinic wavelengths in the range of lambda =11-13 nm, appears to be the natural extension of optical lithography into the NGL-regime. All-reflective imaging systems, utilizing aspherical, multilayer coated, near normal incidence mirrors with reflectivities around 70%, were designed to enable diffraction limited imaging from 70nm down to 30nm feature sizes at high throughput. However, diffraction limited performance of optical systems calls for surface figures of typically lambda /50 to lambda /100. Considering the operation wavelength of lambda =11-13 nm, this demand corresponds to unprecedented requirements for figure metrology and fabrication technology. Simultaneously, low flare levels and high reflectivities of the mirrors have to be ensured for proper system operation. These properties are related to the substrate roughness in the mid spatial (typically 1 mum-1mm) and high spatial (typically <1<mu>m) frequency ranges respectively. Accordingly these quantities have to be generated and controlled on a few- Angstrom -level. In this paper, the metrology and fabrication concepts at CARL ZEISS will be reviewed. The present status in the fabrication of specific EUVL mirrors will be reported as well.