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Electron Beam LithographyElectron Beam Lithography

FYSZ460Advanced Laboratory Exercise

Mikko Palosaari

mikko.palosaari@jyu.fi

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The Objective of the Laboratory Exercise

To give an introduction to

• The operation of a Scanning Electron Microscope (SEM)

• Electron Beam Lithography (EBL)• Working in laboratory and in cleanroom

conditions

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About electron microscopes

First one developed in 1930s and the first commercial one in 1965.

The light used in optical microscopes is substituted with very thin beam of electrons (0.4 nm-5 nm).

First commercially produced 100 kVSEM by Siemens

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About electron microscopes

With light microscopy the resolution is approximately limited with the so-called Abbe criteria due to diffraction of light

With electrons the wavelength is so small that their wavelength does not limit imaging.

nm 200sin 2

n

d

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About electron microscopes

SEM: Conducting samples

TEM (Transmission electron microscope):

Samples have to be thin enough so that the electrons can transmit through.

Operated in vacuum

Less scattering of electrons

Only for solid state samples, e.g. biological samples have to be dried and coated.

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Scales and magnifications

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Generation of Electron beam

Electrons produced by electron gun (field emission, thermionic emission)Emitted electrons accelerated by high voltage (~ 10kV – 100 kV)e-beam focused by magnetic lensesBeam scanned over the sample by deflection coilsScattered (transmitted) electrons detected

Courtesy of Iowa State Univ.

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About the interaction of the electron beam and the sample

Depending both on the sample and the detector different kind of information obtained.

Information from the different layers of the sample.

Different kind of detectors needed.

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Interactions of the electron beam and the sample

Backscattered electrons

Secondary electrons

X-rays Cathodoluminesecne

Transmitted electrons

Auger electrons

Specimen

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Interactions of the electron beam and the sample

a) Incident electron knocks out an inner shell electron of the target atom.

b) Outer shell electron fills the vacancy in the inner shell and this transition emits X-rays.

These X-rays can be used to characterize the target material since every element has its unique characteristic X-rays.

X-rays Backscattered electrons

Secondary electrons

X-rays Cathodoluminesecne

Transmitted electrons

Auger electrons

Specimen

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Interactions of the electron beam and the sample

a) Incident electron knocks out an inner shell electron of the target atom

b) Outer shell electron fills the vacancy in the inner shell and simultaneously another outer shell electron is emitted from the sample.

These Auger electrons can also be used to characterize the target material.

Auger Electrons Backscattered electrons

Secondary electrons

X-rays Cathodoluminesecne

Transmitted electrons

Auger electrons

Specimen

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Interactions of the electron beam and the sample

a) Cathodoluminescence occurs when electron beam promotes electrons from the valence band into the conduction band, leaving behind a hole.

b) When an electron and a hole recombine, it is possible for a photon to be emitted. This is called cathodoluminescence.

c) Only with non-metallic materials.

Cathodoluminescence Backscattered electrons

Secondary electrons

X-rays Cathodoluminesecne

Transmitted electrons

Auger electrons

Specimen

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Interactions of the electron beam and the sample

a) Electron interacts with target material nucleus.b) Energy of BEs much higher than that of SEs

→ information from deeper layers.c) Number and scattering direction of BEs determined

by the atomic number and the incident angle of the e-beam.

d) Can be used to detect composition difference.

Backscattered Electrons Backscattered electrons

Secondary electrons

X-rays Cathodoluminesecne

Transmitted electrons

Auger electrons

Specimen

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Interactions of the electron beam and the sample

a) The number of SEs depends greatly on the incident angle of the electron beam to the specimen surface

b) SEs are low in energy -> emitted only from the surface

c) Most suitable signal for observing surface topography

Secondary ElectronsBackscattered electrons

Secondary electrons

X-rays Cathodoluminesecne

Transmitted electrons

Auger electrons

Specimen

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Image formationElectron beam scanned over the sampleInformation emitted from each scanned pointThe signal from the detector (e.g. the number of SEs) amplified and fed into CRT (cathode-ray tube, nowadays often computer screen)On the CRT the brightness is controlled according to the signal strength as a function of the position of electron beam on the sample

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Detectors

SE: collected by “post acceleration voltage” applied to scintillator

→ shadowless illumination image formed

BE: semiconductor detector, image formed by the electrons emitted towards the detector

→ one-side illumination image formed

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Sample preparation

Surface must be cleanSEM observes the surface layer of specimen

Morphological construction must be maintainedE.g. Biological samples must be dried and emulsion

specimens may be frozen

Specimen must not acquire an electrostatic chargeMetal coating, low voltage observation or low vacuum may

be used.

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Features of the SEM+ High resolution+ High contrast+ High depth of field/focus, large focal depth

- Charging up effects- Aberrations (Spherical, chromatic, diffractions)- Astigmatism- Sensitivity to vibrations etc. external factors- Vacuum

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4.4.2010 High depth of field example. Different plant pollen.

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False color SEM images

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SEMs in NSC

Raith e-LiNEEBL feature size < 20 nmSEM imaging resolution: <10 nm

LEO 1430EBL feature size ~ 200 nmSEM imaging resolution: <100 nm

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Electron beam lithography

EBL used in research and in specialized tasks.

Smaller line width than with UVL.

Slow(er), serial exposure.

Flexible.

Industrial application: mask making for UV lithography.

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Lithography steps

Substrate

2-layer resist: PMMA, P(MMA-MAA)

1. Resist Coating2. e-beam patterning

3. Development4. Metal deposition5. Lift-off

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Resists

Two tasks• To react to radiation• To protect the surface

Three components• Film forming• Sensitive to radiation• Solvent

Positive vs. negative resist

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ExposureOptical

-Mask required

-Parallel

-Limited by wavelength of light, diffraction and resist properties

EBL– Serial, point by

point– Not limited by

wavelength of electrons

– Limited by resist properties

EBL exposure is governed with the formula:

=dose ✕ exposed areabeam current ✕ exposure time

Total charge of incident electrons = step size

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DevelopingChip immersed to the developer chemical.Exposed resist with smaller molec. weight dissolves more readily.Undercut profile.

Other process steps: Metal coating/deposition Lift-off

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Ultraviolet lithography

UV lithography most commonly used in industry.Fast, parallel exposure.Suitable for mass production.Commercial products: line width 90 nm (less than 60nm ?),Highly complicated & specified optics required for the state of the art methods.With EUV even sub 20 nm line widths demonstrated.

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Practical issuesCourse website http://users.jyu.fi/~mirejupa/ebl/

Study material– Invitation to the SEM World link at the website– JEOL Guide to Scanning Microscope Observation link at the

website– Marc J. Madou: Fundamentals of Microfabrication the Science of Miniaturization

2nd ed.– Sami Franssila: Introduction to Microfabrication– Ivor Brodie and Julius J. Muray: The Physics of Micro/Nano –Fabrication

Homework from website

Design CAD file

Schedule for the practical part

Report (will be graded)

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CAD image

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Ready sample