Electron Beam Lithography (EBL) is a specialized technique for creating the extremely fine patterns required by the modern electronics industry for integrated circuits. This is possible due to the very small spot size of the electrons, whereas the resolution in optical lithography is limited by the wavelength of light used for exposure. The electron beam has wavelength so small that diffraction no longer defines the lithographic resolution.
How Does Electron Beam Lithography Work?
Derived from the early scanning electron microscopes, the technique in brief consists of scanning a beam of electrons across a surface covered with a resist film sensitive to electrons, thus depositing energy in the desired pattern on the resist film.
Processing Electron Beam Resists
Electron beam resists are the recording and transfer media for e-beam lithography. The usual resists are polymers dissolved in a liquid solvent. Liquid resist is dropped onto the substrate, which is then spun at 1000 to 6000 rpm to form a coating. After baking out the casting solvent, electron exposure modifies the resist.
Positive Tone and Negative Tone - the Two Forms of Electron Beam Resists
As in optical lithography, there are two types of e-beam resists: positive tone and negative tone, with the usual behaviour, i.e., positive resists develop away at exposed regions, whereas in the case of negative resist the developed region remains after development.
Using Polymethyl Methacrylate (PMMA) as Positive Electron Beam Resists
Polymethyl methacrylate (PMMA) is the standard positive e-beam resist, usually purchased in two high molecular weight forms (495K or 950K) in a casting solvent such as chlorobenzene or anisole. We make use of 950 PMMA, 4% in anisole. Electron beam exposure breaks the polymer into fragments (as shown in figure 1) that we dissolve in a 1:1 MIBK:IPA developer (MIBK is Methyl Isobutyl Ketone and IPA is Isopropyl Alcohol).
Figure 1. Electron beam exposure breaking the polymer into fragments.
The Component Parts that Make Up an Electron Beam Lithography System and the Elphy Quantum Product
The Materials and Microsystems Laboratory’s e-beam lithography system, called Elphy Quantum from Raith Gmbh, was delivered in April 2000. The Elphy Quantum is a universal lithography system which consists of a scan generator electronic (hardware) and a PC-based operating software. The system has the control in three major areas of Scanning Electron Microscopes (SEM): Beam Blanker control, Scan & Signal control and Stage control. Elphy Quantum is a Windows-based operating software and its functionality is based upon a modular design. Editing and pattern design is made simple with a GDSII internal editor. This allows users to build hierarchy patterns on different levels and designs with any dose level. Then pattern data can be generated with the simple CAD program included, or can be imported from a DXF (Auto CAD) file.
Industry Applications for Electron Beam Lithography
Application areas of e-beam lithography span a wide range from cryo-electric devices, opto-electronic devices, quantum structures, transport mechanism studies of semiconductor/superconductor interfaces, microsystem techniques, optical devices.
Using Silicon-Based Structures for Photonic Applications
Currently, our interest is principally focused in direct applications for the realization of silicon-based structures for photonic applications. Photonic crystals can be created in semiconductors using standard nanofabrication technology (electron beam lithography and dry-etching), and they are interesting to realize optoelectronic structures.