ABSTRACT
The relentless drive of the semiconductor industry to smaller device dimensions continues to present new challenges for all the enabling fabrication technologies, and in particular to microlithography. At present, the most advanced microelectronic devices have dimensions of 0.18 µm, and it is expected that these will be reduced further within the next few years to 100 nm and below. The mainstay of mass production lithography has been projection optical lithography, in which the pattern on a photomask is imaged in reduction onto the silicon wafer. From fundamental principles we know that the resolution in such a configuration is directly proportional to the wavelength. Thus, the most straightforward way of reducing printed dimensions is to use radiation sources at shorter wavelengths. Several years ago the industry made the transition from a continuous-wave mercury discharge lamp at 365 nm to a pulsed excimer laser operating at 248 nm (KrF lasers). It is widely expected that in the near future a similar transition will take place to 193 nm (ArF lasers). Beyond that, 157 nm (F2 lasers) is being explored for sub-100-nm lithography. This shift in lithographic wavelengths from the nearultraviolet (UV) to deep-UV and eventually to vacuum-UV (VUV) poses new challenges related to optical materials, optical coatings, detectors, and ambient control. This chapter summarizes the key issues encountered at the shorter lithographic wavelengths, and the present state of the art in addressing them.
