In addition to EUV lithography, a number of other alternatives to optical lithography have been or are being pursued. These alternative techniques have sometimes been called next-generation lithographies, and are frequently referred to by the acronym NGL. A number of alternatives to optical lithography have been conceived, but none has yet been developed to the point that it is ready for implementation in manufacturing. Several of these alternative lithographic techniques - proximity x ray, electron-beam direct write, electron projection, ion-projection lithography, nanoimprint lithography, and directed self-assembly - are discussed in this chapter. Each of these approaches has technical challenges that must be overcome before they will be usable in semiconductor manufacturing. In this chapter, the basic concepts underlying several of these technologies are discussed, and the challenges that need to be addressed are highlighted.
13.1 Proximity X-ray Lithography
Optical lithography is limited by diffraction, which is most significant when objects are comparable in size to the wavelength of light. This fact of physics has driven decreases in the wavelength of light used in optical lithography. Similarly, the lithographic use of wavelengths in the x-ray portion of the electromagnetic spectrum was motivated by the idea that diffraction effects could be effectively neutralized by using photons with extremely short wavelengths. However, at x-ray wavelengths there are no known materials for making image-forming lenses or mirrors. Consequently, x-ray lithography involves the use of proximity printing, where the mask is brought to within a few microns of the wafer and the x rays are passed directly through the mask and onto the wafer (Fig. 13.1). This is in contrast to optical lithography, which has the potential for projection of the image by a lens.
Since there are no materials that are highly transparent, x-ray masks are made on very thin membranes (thickness <2 μm) comprised of low-atomic-number materials, on which the circuit patterns are placed in the form of high-atomic-number material (Fig. 13.1). A large percentage of the x rays pass through the low-atomic-number material, but the x rays are absorbed or scattered by the high-atomic-number materials, thus generating pattern contrast.