Electron Beam Lithography (EBL)

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Electron Beam Lithography

Electron Beam Lithography (EBL) refers to a lithographic process that uses a focused beam of electrons to form the circuit patterns needed for material deposition on (or removal from) the wafer, in contrast with optical lithography which uses light for the same purpose. Electron lithography offers higher patterning resolution than optical lithography because of the shorter wavelength possessed by the 10-50 keV electrons that it employs.

Given the availability of technology that allows a small-diameter focused beam of electrons to be scanned over a surface, an EBL system doesn't need masks anymore to perform its task (unlike optical lithography, which uses photomasks to project the patterns). An EBL system simply 'draws' the pattern over the resist wafer using the electron beam as its drawing pen. Thus, EBL systems produce the resist pattern in a 'serial' manner, making it slow compared to optical systems.

A typical EBL system consists of the following parts: 1) an electron gun or electron source that supplies the electrons; 2) an electron column that 'shapes' and focuses the electron beam; 3) a mechanical stage that positions the wafer under the electron beam; 4) a wafer handling system that automatically feeds wafers to the system and unloads them after processing; and 5) a computer system that controls the equipment.

Figure 1. Example of an electron beam

lithography equipment from Jeol

The resolution of optical lithography is limited by diffraction, but this is not a problem for electron lithography. The reason for this is the short wavelengths (0.2-0.5 angstroms) exhibited by the electrons in the energy range that they are being used by EBL systems. However, the resolution of an electron lithography system may be constrained by other factors, such as electron scattering in the resist and by various aberrations in its electron optics.

Just like optical lithography, electron lithography also uses positive and negative resists, which in this case are referred to as electron beam resists (or e-beam resists). E-beam resists are e-beam-sensitive materials that are used to cover the wafer according to the defined pattern.

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