e sc 521 pattern generation at the nanoscale wook jun nam · advanced lithography techniques:...

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


Electron Emitter (Gun)-continued


• 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).



• 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.



• 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)


Liquid Metal Ion Source (LMIS)

Ion Emitter (Gun)-continued


• 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


• 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).

e sc 521 pattern generation at the nanoscale wook jun nam · advanced lithography techniques:...

Documents