E SC 521
Pattern Generation at the Nanoscale
Wook Jun Nam
Unit 2
Advanced Lithography
Techniques: Direct Writing
Lecture 1A
Electron Beam Lithography I
Outline
• Ebeam Lithography (EBL) System
Ebeam Lithography System (EBPG-5200)
http://www.vistec-litho.com/news/news-detail/vistec-advanced-ebpg5200-electron-beam-lithography-system-ordered-by-penn-
state/249b94b5b330b6a3d366a52122d444ae/?L=atckytfhsxgqn
Ebeam Lithography System (EBPG-5200)
Nanofabrication: Principles, Capabilities, and Limits, Zheng Cui, Springer (2008)
Electron Emitter (Gun)
Thermionic cathode
Field Emission (FE) cathode
Nanofabrication: Principles, Capabilities, and Limits, Zheng Cui, Springer (2008)
Electron Emitter (Gun)-continued
Nanofabrication: Principles, Capabilities, and Limits, Zheng Cui, Springer (2008)
• The size of the source beam is very important to achieve
images with good resolution.
• Thermionic cathode generates non-focused e-beam. A
strong lens located in front of the emitter source is
required to reduce the source area (cross-over spot).
• Reducing the source size at cross-over has limitation due
to the Coulomb repulsion among the highly concentrated
electrons.
• Field emission (FE) source extracts electrons from a
material applying extremely high electric field ( > 108 V/m).
Electron Emitter (Gun)-continued
Nanofabrication: Principles, Capabilities, and Limits, Zheng Cui, Springer (2008)
• Very high electric field is created at the sharp end of tip
with relatively low bias application.
• Because electrons only emit at the points where the
electrical field exceeds threshold strength, the emission
area can be finely controlled and very small.
Electron Emitter (Gun)-continued
Nanofabrication: Principles, Capabilities, and Limits, Zheng Cui, Springer (2008)
Electron Emitter (Gun)-continued
Nanofabrication: Principles, Capabilities, and Limits, Zheng Cui, Springer (2008)
• Cold FE offers the best beam characteristics (e.g., beam
size, emission current density).
• The low operation temperature (~300K) allows absorption
of gases on the emitter tip, and it needs regeneration step,
applying heat in the tip, in every 6~8 hours of operation.
• E-beam lithography relies on stable emission current to
deliver consistent exposure dose. Any emission current
drift or noise can seriously deteriorate the quality of resist
exposure. This has been the reason to develop thermal
FE cathodes for e-beam lithography systems.
Ion Emitter (Gun)
Nanofabrication: Principles, Capabilities, and Limits, Zheng Cui, Springer (2008)
Liquid Metal Ion Source (LMIS)
Ion Emitter (Gun)-continued
Nanofabrication: Principles, Capabilities, and Limits, Zheng Cui, Springer (2008)
• A tungsten wire of about 0.5-mm diameter is
electrochemically etched into a needle with the tip radius
of only 5–10 mm.
• The tungsten needle is then wetted with molten metal.
• Gallium is the most used liquid metal material because of
its low melting temperature (29.88C).
• A small amount of liquid metal adheres to the needle.
When a high voltage is applied, an electrostatic force
exerts on the liquid metal, pulling it into an extremely small
apex.
Ion Emitter (Gun)-continued
Nanofabrication: Principles, Capabilities, and Limits, Zheng Cui, Springer (2008)
• The extremely high field, metal atoms at the liquid apex
become ionized and escape from the liquid metal surface
in the form of field evaporation, resulting in ion emission.
• Liquid metal ion source is a FE ion source. Like the FE
electron sources mentioned earlier, LMIS has high
emission current density (high brightness) and extremely
small size of emission area.
Ebeam Lithography System (Computer Screen)
Holders
3 inch wafer holder
Holder (Loadlock Chamber)
Holder (Marker Blocks & Faraday Cup)
Holder (Holder Definition)
Aligner
Aligner (continued)
• Used for combining ebeam with other types of lithography.
• Alignment accuracy: achievable (SEM mode)
• Alignment markers:
– squares, octagons, and crosses
– 1um deep etched Si pits
– Metals (e.g., Au, Pt, W)
http://www.intechopen.com/books/lithography/high-energy-electron-beam-lithography-for-nanoscale-fabrication
Aligner (continued)
Center of the ebeam pattern
already existed structures
Identify the location of the center
of the ebeam pattern
(reference origin: Faraday cup)
Ebeam pattern (red)
SEM Mode
Ebeam writer has a very similar tool configuration to a SEM,
so it can be used as a SEM !!!
Summary
• There are two types of electron emitters, and they are thermionic cathode and field emission (FE) cathode.
• Thermionic cathode needs a strong lens to form cross-over of the electron beam.
• Cold FE provides the best beam characteristics (e.g., small beam radius, high emission current density), but it is not used for ebeam writer because of its requirement of regeneration in every 6~8 hours.
• Liquid metal ion source (LMIS) operates similar to electron FE source, and offers high emission current density (high brightness) and extremely small size of emission area.
Summary (continued)
• Ebeam lithography holders has a faraday cup and markers.
• The faraday cup and markers are used for creating spot table.
• There are two types of aligners (optical microscope type and SEM modes), and they are used when ebeamlithography pattern is aligned with the patterns formed by other lithography techniques (e.g., optical lithography).