Because the resolution capability of optical lithography is fundamentally limited by the phenomenon of diffraction, work is ongoing to develop alternative lithography technologies that can support the continuation of Moore's Law past the diffraction limit that was estimated in Chapter 10. These alternative techniques are often 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 next-generation lithographic techniquesâproximity x-ray, extreme ultraviolet (EUV), electron beam and optical direct write, electron projection, and ion-projection lithographyâare 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.
12.1 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 use for lithography 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. 12.1). 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 comprised of very thin membranes (thickness < 2 Î¼m) comprised of low-atomic-number materials, on which the circuit patterns are placed in the form of high-atomic-number material (Fig. 12.1). A large percentage of the x rays pass through the low-atomic-number material, but the x rays are generally absorbed or scattered by the high-atomic-number materials, thus generating a pattern contrast. Silicon carbide is a typical membrane material, and silicon nitride films were used early in the development of x-ray lithography.
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