Reflective photomasks for sub-quarter-micron lithography.

摘要

Decreasing minimum feature sizes for integrated circuit (IC) fabrication require corresponding reductions in wavelength for projection lithography systems used to pattern these devices. As operating wavelengths decrease into the DUV, 248nm wavelength and shorter, the number of suitably transparent refractive materials drops dramatically, complicating the task of optical design, especially for reduction configurations. One can however find suitable reflective materials for the DUV, and this suggests the use of reflective optics for DUV lithographic systems, provided a suitable mirror based design can be obtained.;This study focuses on the design, fabrication and testing of novel reflective photomasks, which form an essential optical component for the mirror-based, unity-magnification Markle-Dyson (M-D) projection optical system. A 1/6 scale prototype M-D system was assembled and tested. This system is nearly aberration free by design, and requires only one refractive material, fused silica. Thus high numerical aperture (NA) low wavelength operation can be achieved with this optical design provided that a novel illumination path and a reflective mask are employed. Such a system, with an NA of 0.7 and 248nm illumination, has quarter-micron resolution.;The reflective mask design investigated uses a fused silica substrate with a patterned reflective film, behind which is an absorbing non-reflective layer. The reflective mask uses the internally reflected light from the film/silica interface. Films of aluminum, silicon and chromium were investigated for the reflector material. Of these materials, amorphous silicon provided the most appealing combination of high reflectivity and well behaved etching characteristics. Novolak photoresist material was used as the absorber material.;Reflective masks with line and space features down to 0.125