What’s common among optoelectronic devices, cryo-electric devices, microsystems, optical devices, and quantum structures? Electron-beam lithography is used in all of them.
Over the years, the application areas of electron lithography have grown. In this post, you’ll learn about the electron-beam lithography system and how it works.
What is an Electron-Beam Lithography System?
An electron-beam lithography system enables you to scan a focused beam of electrons on a surface. Usually, these surfaces are covered with an electron-sensitive resist. You can focus the beam of electrons to display custom shapes on the surface.
When it comes to commercial use, companies use dedicated e-beam writing systems. These systems are expensive and can cost more than $1 million. However, for research purposes, institutions often convert electron microscopes into EBL systems. The whole setup costs less than $100k.
How Does an Electron-Beam Lithography System Work?
An electron-beam lithography system allows you to draw a custom pattern on a surface coated with electron resist. When exposed to electrons, the material becomes soluble or cross-links, leading to its removal by immersion or resistance to the solvent.
When compared with other self-assemble, stamping, or photolithography methods, EBL can be slower and more expensive. Besides, it needs tidy room facilities. Hence, it’s ideal for high-resolution patterns for which photomask creation is not feasible.
Let’s take a quick look at the working process of an EBL system.
1. Electron Source
Most systems use hot W/ZrO2 as an electron source. The electrons are transmitted through electron emission using magnetic or electrostatic lenses. The resist layer configuration creates shapes like stepped or T-shape.
The surface comprises multiple layers of resist with varying sensitivities to electron beams. When exposed to electrons, these layers adjust their solubility by undergoing secondary electron production, diffusion, and scattering.
Materials that are exposed to the electron beam are electrically grounded. This helps avoid charging effects, which can adversely affect the electron lithography process. Engineers can achieve grounding by adding a thin layer of gold or aluminum on the top of the resist or between the resist and substrate.
Components of an EBL System
Engineers consider the following aspects before working with an electron-beam lithography system.
1. Electron Sources
As discussed, most systems use W/ZrO2 as an electron source. However, it’s ideal for systems with high-resolution requirements, as they need field electron emission sources.
In the case of lower-resolution systems, thermionic sources can be used. These systems use lanthanum hexaboride as the election source.
When it comes to source selection, most engineers prefer thermal field emission sources over cold sources. While thermal sources have a larger beam size, they provide improved stability for longer hours.
Both magnetic and electrostatic lenses are commonly used in electron-beam lithography systems. Electrostatic lenses are less common due to their high deviation, which impedes fine focusing. Since magnetic lenses enable finer focusing, they’re used over electrostatic lenses.
As of now, no mechanism exists that allows us to develop achromatic beam lenses. Once developed, these lenses would provide very narrow dispersions for fine focusing.
3. Stage, Stitching, and Pattern Overlay
For small beam deflections, companies use electrostatic deflection lenses. For larger deflections, electromagnetic scanning is used. The writing field in the process is 100 micrometer – 1 mm of order. Medium-large-sized patterns need stage moves. An accurate staging ensures stitching and pattern overlay.
4. Electron-Beam Write Time
Electron-beam write team refers to the minimum time required to expose a provided area for a specified dose. Here’s the formula for the same:
D.A = T.I
Here, D is the dose, A is the area exposed, T is the time taken to expose the object, and I is the beam current.
5. Shot Noise
The effect of shot noise becomes prominent when the number of elections at fixed-dose reaches ~10,000. When the feature size shrinks, it results in the shrinkage of the number of incident electrons. The shot noise effects result in major natural dose variations within an enormous feature population.
How Long Does the Installation of Electron-Beam Lithography Systems Take?
Several factors come into play when it comes to determining the time required to install electron lithography systems. For commercial use, the systems used are high-resolution and require precise layer resist formation. This takes more time.
In the case of research or educational use, low-resolution works well. Researchers can convert an electron microscope into an EBL system, which doesn’t require much time.
In all, the time needed to implement an EBL system depends on the complexity of the process, components needed, and the installer’s expertise.
The ability of electron-beam lithography systems to draw custom patterns at both low and high resolutions makes them a popular choice in the production of various types of devices. However, the output achieved generally has a high resolution and low throughput. This limits EBL systems’ use to research & development, education, and production of semiconductor devices.