Proximity Effect in Electron Beam Lithography
A fully PC controlled FE SEM intended for both – for high vacuum as well as for low vacuum operations. Outstanding optical properties, flicker-free...
Electron Beam Lithography (EBL) is a technique frequently used to prototype optical surface structures such as gratings, waveguides or photonic crystals. These structures typically consist of micro- or nano- sized features that are patterned over relatively large areas (compared to the size of the features) with high structural density. A key issue in the fabrication of such structures is the requirement for high accuracy. Even very small deviation (a few nanometers) from a desired pattern leads to unwanted optical losses. There are several factors that can affect the accuracy of the features, one of which is the proximity effect. In this application example we present the TESCAN DrawBeam software module with integrated proximity effect correction.
The proximity effect (PE) is predominantly a result of back-scattered electrons (high kinetic energy electrons), which are reflected from the substrate back into the resist layer above, causing exposure of the resist away from the original region of the incidence beam. The backscattered electrons originate from collisions with atoms in the substrate and travel in the resist at wide angles compared to electrons in the primary beam. The amount of backscattered electrons, and thus the severity of the proximity effect, depends strongly on the accelerating voltage and the substrate composition.
The proximity effect can be quantitatively described with a proximity function by the sum of two superimposed Gaussian distributions (one distribution corresponds to the forward scattering of electrons – the Forward scattering parameter in DrawBeam and the second corresponds to the backward scattering of electrons – the Backward scattering parameter in DrawBeam). This approximation (proposed by Chang1 in 1975) assumes that the proximity function has the same shape as the energy distribution of the electrons absorbed in a material.