Optical projection lithography

Regarding a periodic fine structure assembly consisting of lines and spaces, the minimum line resolution of the pattern in terms of the minimum achievable feature size, LWmm, can be estimated with the aid of Eq. (9-1)  

Actually, LWmin is equal to p/2. Here, p denotes the pitch, i.e. the distance made up of a pair of lines and spaces. A is the wavelength of the exposure light, and ki is a system factor that depends on various parameters such as resist response, pattern geometry in the mask, etc. NA is the numerical aperture given by Eq. (9-2). 

A different approach, whereby the resolution may be improved by 50-100%, is based on the use of phase-shifting transmission masks. The latter contain opaque regions, as conventional masks do, but some of the apertures are covered with a transparent phase-shifting material, which reverses the phase of the light passing through them. The interaction of phase-shifted with non-phase- 

Imprinting lithography is another maskless technique capable of generating sub-100 nm patterns. It is essentially a nanomolding process, in which a transparent patterned template is pressed into a low-viscosity monomer layer dispensed onto the surface of a wafer. Thereby, the relief structure of the template is fiUed. After photopolymerization of the monomer with the aid of UV light (see Chapter 10), the template is separated, leaving a solid polymer replica of the template on the surface of the wafer. With the aid of subsequent etching processes, the pattern is fixed on the wafer s surface [4]. 

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Projection optical lithography has been the mainstream technology in the semiconductor industry for the last two decades [2]. Figure 9.2 shows a schematic depiction of an optical projection system consisting of a laser light source, a mask, a projection lens, and a resist-coated wafer. The projection of the pattern of the mask onto the resist layer provides a demagnification ratio of up to 4x. 

M. Rothschild, Projection Optical Lithography, materials today 8 (2005) 18. 

The evolutionary refinement of projection optical lithography since its introduction can be imderstood by consideration of these equations. The overall goal is improved resolution while maintaining a maximum DOF. Resolution improvements may be realized by increasing the NA, by redncing the exposure wavelength k, or by process and tool refinements that rednce the value of ki. There are practical limits to each of these approaches, however. 

Rothschild, M. (2005) Projection optical lithography. Mater. Today, 8,18. 

Preliminary experiments with electron-beam writing and ion-beam projection lithography have demonstrated that the S-layer may also be patterned by these techniques in the sub-lOO-nm range (nnpnblished resnlts). The combination of ion-beam projection lithography and S-layers as resist might become important in the near fntnre, since ion beams allow the transfer of smaller featnres into S-layer lattices compared to optical lithography. 

We have now discussed qualitatively several concepts which are important in developing an understanding of image projection in optical lithography. These are summarized below ... 

There are six basic elements of optical lithography, i.e., (i) an exposure source, (ii) an illumination optics system, (iii) a photomask or reticle and its pellicle, (iv) a projection optics system, (v) a wafer stage, and (vi) a photoresist-coated wafer. As shown in Fig. 13.3, the source provides the exposure radiation that, with the aid of the illumination optics, illuminates the photomask and transfers... 

Hollenshead and L. Klehanoff, Modeling extreme ultraviolet/H20 oxidation of ruthenium optic coatings, J. Vac. Sci. Technol. B 24,118 130 (2006) Y. Gomei, H. Takase, T. Aoki, Y. Kakutani, and M. Niihe, Scaling law in acceleration test of extreme ultraviolet lithography projection optics mirror contamination, J. Vac. Sci. Technol. B 23, 2848 2851 (2005) A.N. Broers, W.W. Molzen, J.J. Cuomo, N.D. Wittels, Electron beam fabrication of 80A metal structures, Appl. Phys. Lett. 29, 596 598 (1976). 

Second, because contamination resulting from carbon deposition depends on the local intensity of the EUV radiation at the surface of the mirror, it is possible that certain parts of the mirror surface in the projection optics may experience a local reduction in reflectivity that is larger than that occurring at other locations. The result is apodization of the projection optics, which can cause CD variation across the chip that is printed with EUV lithography. ... 

Liddle, L.R. Harriot, A.E. Novembre, and W.K. Waskiewicz, SCALPEL a projection electron beam approach to sub optical lithography, http //www.bel labs.com/proJect/SCALPEL. Levinson, Principles of Lithography, 2nd ed., p. 396, SPIE Press, Bellingham, WA (2005). 

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