Optical lithography has been successfully used since the 1960s for integrated-circuit production at linewidths of several microns. At present, volume production tools operating at 193-nm wavelength deal with linewidths about 100 nm. Intel's cofounder Gordon E. Moore predicted this downscaling trend in 1965, and the leading chip manufacturers have followed “Moore's Law” during the past and plan to follow it for the coming decade with resolutions below 35 nm. According to Rayleigh's criterion, the resolution of lithography systems is directly proportional to the wavelength λ used and inversely proportional to the numerical aperture (NA) of the system. Consequently, one strategy for next-generation lithography (NGL) toward higher resolution is to decrease wavelength.
Most likely, NGL optics will be based on extremely short wavelength UV light, also called soft-x-ray or EUV light, at a preferred wavelength of 13.5 nm. Other NGL options such as electron projection lithography, maskless lithography, and imprint lithography are still subjects of major research programs, but an insurmountable obstacle for these technologies is their low throughput prospects.
In the case of EUVL, the very low EUV transparency of common materials implies a technology migration from transparent lenses (all-refractive optics) to mirrors (all-reflective optics). Accordingly, any EUV chip manufacturing tool (wafer scanner) will need a powerful light source and efficient reflective optical components in order to supply a sufficient number of photons for a stable resist exposure process.
The main optical components of an EUVL scanner are shown schematically in Fig. 33.1. EUV light emitted from a suitable source is collected by the source optics and projected into an aperture (intermediate focus or IF), where it enters the illumination system. A uniformly illuminated field of the reticle (mask) is then imaged by the projection optics onto the resist-coated wafer. In the region of the IF an enlarged image of the source itself is created by the collector optics.
High EUV reflectivity of optical surfaces, as required for such an application, can be achieved either by suitable multilayer coatings at or close to normal incidence, or by very smooth metal reflectors at grazing incidence.
Several technological approaches, including plasma emission and synchrotron radiation, have been taken toward the development of a high-power EUV light source for lithography applications. Currently, the most promising EUV source types are the electrical-gas DDP and the LPP. Among other wavelengths, both types of sources emit light at around 13.5 nm into solid angles of around 2π sr or even more. For details about DPP and LPP sources, see Chapters 3 and 4, respectively. Information on some alternative sources other than DPP or LPP can be found in Chapter 6.