Electron Beam Lithography (EBL)

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.

Positive electron resists produce an image that is the same as the pattern drawn by the e-beam (positive image), while negative ones produce the reverse image of the pattern drawn (negative image). Positive resists undergo bond breaking when exposed to electron bombardment, while negative resists form bonds or cross-links between polymer chains under the same situation.

As a result, areas of the positive resist that are exposed to electrons become more soluble in the developer solution, while the exposed areas of the negative resist become less soluble. This is the reason why positive resists form positive images - because its electron-exposed areas will result in exposed areas on the wafer after they've dissolved in the developer. In the case of negative resists, the electron-exposed areas will become the unexposed areas on the wafer, forming a negative image.

The resolution achievable with any resist is limited by two major factors: 1) the tendency of the resist to swell in the developer solution and 2) electron scattering within the resist.

Resist swelling occurs as the developer penetrates the resist material. The resulting increase in volume can distort the pattern, to the point that some adjacent lines that are not supposed to touch become in contact with each other.

Resist contraction after the resist has undergone swelling can also occur during rinsing. However, this contraction is often not enough to bring the resist back to its intended form, so the distortion brought about by the swelling remains even after rinsing. Unfortunately, a swelling/contraction cycle weakens the adhesion of the smaller features of the resist to the substrate, which can create undulations in very narrow lines. Reducing resist thickness decreases the resolution-limiting effects of swelling and contraction.

When electrons strike a material, they penetrate the material and lose energy from atomic collisions. These collisions can cause the striking electrons to 'scatter', a phenomenon that is aptly known as 'scattering'. The scattering of electrons may be backward ( or back-scattering, wherein electrons 'bounce' back), but it is often forward through small angles with respect to the original path.

During electron beam lithography, scattering occurs as the electron beam interacts with the resist and substrate atoms. This electron scattering has two major effects: 1) it broadens the diameter of the incident electron beam as it penetrates the resist and substrate; and 2) it gives the resist unintended extra doses of electron exposure as back-scattered electrons from the substrate bounce back to the resist.

Thus, scattering effects during e-beam lithography result in wider images than what can be ideally produced from the e-beam diameter, degrading the resolution of the EBL system. In fact, closely-spaced adjacent lines can 'add' electron exposure to each other, a phenomenon known as 'proximity effect.'

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