Anisotropic Organic/Inorganic Resists: A Novel Concept for Electron Proximity Effect Reduction
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industry will prevail as it is now widely accepted that the shorter the radiation wavelength the finer the theoretical resolution, thus making Extreme UV, X-ray, electron beam and ion beam lithographies more promising candidates. Electron beam lithography is probably the less risky, the most accepted and the most versatile technique benefiting from the major R&D knowledge accumulated over the past 50 years. Companies such as IBM, Hitachi and Siemens [2-3] have already demonstrated that the industrial concept of electron projection lithography (EPL) is technically feasible and viable in terms of throughput since it is a parallel process. With the advent of a new EPL technique called SCALPEL (SCattering with Angular Limitation Projection Electron Lithography) developed by Bell Labs-Lucent Technologies, there is now a real platform for sub-70 nm lithography providing that one of the major problems, namely the electron proximity effect [4], is eliminated. Indeed, this effect translates into severe degradation of the pattern definition as the uniform exposure by the incident beam gives rise to a nonuniform distribution of actually received exposure in the pattern area. Moreover, it has been mathematically proven that no rigorous solution of the proximity equation [5] exists for isotropic resists like PMMA, thus showing that the fundamental problem essentially lies in the electronpolymer microscopic interactions [4] and not in the technological advances of the already sophisticated EPL system itself During the past two decades, experimental attempts [6,7] to limit the electron proximity effects in isotropic resists have not led to any breakthrough, thus proving that the current resists used by the industry are not ideal for the promising sub-100 nm EPL. 97 Mat. Res. Soc. Symp. Proc. Vol. 584 © 2000 Materials Research Society
NOVEL CONCEPT FOR ELECTRON PROXIMITY EFFECTS REDUCTION Current methods for reduction of electron proximity effects The electron proximity effect due to scattering of incident and secondary electrons leading to a pear-like structure [4] in standard organic resists, which degrades the overall resolution of the lithographic process, is too often macroscopically modelled by the following simplistic proximity function (effective exposure) [6,7]: 3 f(r) = k [exp(-r 2/of 2) + TEIV/b2/ b exp(-r 2/13b2)]
(1)
where r is the radial distance from the point of incidence of the electron beam, k is a normalizing constant, qE is the ratio of integrated contributions of backscattered to forward-scattered electrons, and o3f, f3b are the characteristic widths respectively related to forward and backward
scattered electrons. Two methods to reduce the proximity effect have been developed, neither being fully satisfactory. The first and obvious one is to use ultrathin layers (
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