Electron beam lithography
|Electron beam lithography|
|Other Names||e-beam lithography, Ebeam lithography, eBL|
JEOL JBX-6300FS Electron Beam Lithography System|
E-Beam Spinner Hot-Plate Bench 21
E-Beam Solvent Bench 22
|Materials||Si, III-V, Metals, Dielectrics|
Electron beam lithography (e-beam lithography) is a direct writing technique that uses an accelerated beam of electrons to pattern features down to sub-10 nm on substrates that have been coated with an electron beam sensitive resist. Exposure to the electron beam changes the solubility of the resist, enabling selective removal of either the exposed or non-exposed regions of the resist by immersing it in a developer.
The primary advantage of electron beam lithography is that it can write custom patterns with sub-10 nm resolution. This form of direct writing has high resolution and low throughput, limiting its usage to photomask fabrication, low-volume production of semiconductor devices, and research & development.
Method of operation
An Electron beam lithography system uses hardware similar to a scanning electron microscope (SEM) to guide a nanometer sized focused beam of electrons to form a latent image in a layer of resist. The result of this exposure is to render the resist either more soluble (called a positive tone resist) or less soluble (negative tone resist) in an appropriate developer solution. The resulting pattern is then transferred via etching or by depositing other materials. By iterating a number of steps of this type, complex structures of very short length scales can be built up.
Electron beam lithography tools have a certain maximum area that it can write for a fixed stage position know as Write Field. Typically, they range from a few 10s of µms to 1-2 mms. If the pattern to be exposed is more than the size of the write field, the electron beam is blanked, the stage moves by a distance of 1 write field and the writing continues. To avoid discontinuities or overlaps between write fields (known as field stitching errors), an electron beam lithography system has a laser interferometry stage position system that allows stitching of fields with nanometer scale precision.
There is almost always a difference between the digital line width in a pattern and the actual developed feature size after processing. This comes about from electron scattering in the resist. For a well-characterized process, there is generally a fixed difference between the digital size and the actual size, known as bias. This can be implemented by altering all the sizes in the design as appropriate.
Pattern fidelity is limited by electron-matter interactions especially in layouts with a high feature density. The electron scattering within the resist and the from the substrate results in undesired exposures of the resist in regions adjacent to the primary incident beam. This effect is known as the "Proximity Effect" in electron beam lithography. Due to proximity effects, corners in the desired patterns are rounded, gap spacings and linewidths are modified, and certain features may even merge together or disappear completely.
Applications of e-beam lithography span a wide range of nanostructured devices including but not limited to electronic devices, opto-electronic devices, quantum structures, metamaterials, transport mechanism studies of semiconductor/superconductor interfaces, microelectromechanical systems, optical, and photonic devices. E-beam lithography can be also used for mask making and direct writing on non-planar substrates.
- Exposure energy affects resolution, sensitivity and proximity effects
- Exposure dose affects pattern fidelity
- Pattern density affects proximity effects and pattern fidelity
- Resist material affects sensitivity, resolution, and contrast
- Resist thickness affects sensitivity, resolution, and pattern fidelity
- Developer affects sensitivity, resolution and development window
- Development time affects sensitivity, resolution, and exposure window
- Development temperature affects sensitivity, resolution, and exposure window
Figures of merit
Resolution in electron beam lithography is the minimum size of a feature that can be patterned. It depends on the type of the resist used, the thickness of the resist, the type of substrate, and the operating conditions.
The maximum writing area for one field with no stage movement is typically 1-2 mm. If larger areas need to be patterned, they need to be subdivided into fields which are then stitched together using stage movements. Any errors in the butting up of these fields are known as stitch errors. For an electron beam lithography system that has a laser interferometer stage, the stitching errors are between a few nanometers to a few 10s of nms.
Almost all practical devices need several lithography and pattern transfer steps. The process of patterning the different lithography levels relative to each other or a predefined datum is known as Alignment. The mismatch between the different layers of lithography is known as overlay error. For electron beam lithography systems that are able to automatically detect alignment marks, the overlay accuracy is usually 10s of nms. Overlay is strongly dependent on the quality of the alignment marks and the mark detection techniques.
Below is a general description of the electron beam lithography and associated equipment at the LNF.
JEOL JBX-6300FS Electron Beam Lithography System
The JEOL JBX-6300FS is an electron beam lithography system equipped with a thermal field emission electron gun with a ZrO/W emitter. It is capable of writing features down to 8 nm on substrates from 5 mm X 5 mm pieces up to 200 mm diameter wafers.
E-Beam Spinner/Hot-Plate Bench 21
The E-Beam Spinner Hot-Plate Bench 21 is a fully programmable, manual dispense, e-beam resist spinner located in 1480A. It can accept 10 mm X 10 mm pieces to 4" wafers. The bench also has a temperature controlled hot plate for baking/curing substrates.
E-Beam Solvent Bench 22
The E-Beam Solvent Bench 22 is for working with solvents, stripping resist, cleaning wafers, and liftoff. It is considered a non CMOS-clean bench, so wafers with metal are allowed in the bench. It is intended for use with all sized wafers and materials. This bench is located in 1480A.
Point autofocus probe 3D surface mapping microscope used to map out non-planar substrates before performing e-beam lithography on them.
- From Wikipedia.org
- Wiederrecht, Gary. Handbook of nanofabrication. Academic Press, 2010.
- Cabrini, Stefano, and Satoshi Kawata, eds. Nanofabrication handbook. CRC Press, 2012.
- Ren, Liming, and Baoqin Chen. "Proximity effect in electron beam lithography." Solid-State and Integrated Circuits Technology, 2004. Proceedings. 7th International Conference on. Vol. 1. IEEE, 2004.
- Mohammad, Mohammad Ali, et al. Fundamentals of electron beam exposure and development. Springer Vienna, 2012.