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FYS0460 / FYSZ460
Advanced Laboratory Exercise
Electron Beam Lithography
Terhi Hongisto
[email protected]
YN240
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
About electron microscopes
The light used in optical microscopes is ‘substituted’
with very thin beam of electrons
→ smaller wave lenght → better resolution
SEM: conducting samples
TEM: (transmission electron microscope) the samples
have to be thin enough for the electrons to transmitt
 Operated in vacuum
→ less scattering of the electrons
→only for solid state samples (e.g. biological samples
have to be dryed and coated)

Kuva: SEM/TEM/Optical
About the interaction of the electron
beam and the sample

Different interactions
→ 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
Kuva: vuorovaikutukset
About the interaction of the electron
beam and the sample

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Secondary electrons
number of SEs strongly dependent on the incident
angle, low energy
→ surface topography
Backscattered electrons
energy much higher, the number and the scattering
direction of BEs determined by the incident angle
and the atomic number of the substanses composing
the specimen surface
Magnification

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Electron beam scanned over the sample
Information emitted from each scanned point
The signal from the detector (e.g. the number
of SEs) amplified and fed into CRT (cathoderay tube, nowadays often computer screen)
On the CRT the brightness is controlled
according to the signal stength as a function
of the position of electron beam on the
sample
Kuva: Kuvan muodostus
Generation of Electron beam

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Filament (W) heated by current (thermionic
emission source)
Emitted electrons accelerated by high
voltage
e-beam focused by magnetic lenses
Beam scanned over the sample by deflection
coils
Scattered (transmitted) electrons detected
Kuva: Column
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
Features of the SEMs
+ high resolution
+ high contrast
+ high depth of field/focus, large focal depth
- charging up effects
- aberrations (chromatic, )
- astigmatism
- sensitivity to vibrations etc.external factors
-
Kuva: glomerulus
Lithography
UV lithography most commonly used in in industry
-fast, parallel exposure
-suitable for mass production
-commercial products: line width 90 nm (65 nm ?),
highly complicated & specified optics required
EBL in research
-smaller line width
-slow(er), serial exposure
-flexible
-industrial application: mask making for UV
lithography
Lithography
e -beam
patterning
2 -layer
resist
substrate
metal
coating
develop
lift-off
2 layer resist: PMMA, PMMA-MAA (=co-polymer)
metal
Resists
2 tasks
- to react to radiation
- to protect the surface
3 components
- film forming
- sensitive to the radiation
- solvent
Positive vs. negative resist
Exposure
Induces change in the resist
Optical
-Mask required
-Parallel
-Limiting factors
wave length, diffraction
in mask/resist
EBL
-Serial, point by point
-Limiting factors
resist
Dose, cross linking, acceleration voltage, proximity effect
(inter-/ intra-)
Developing

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Chip immersed to the developer chemical
Exposed resist with smaller molec. Weight
dissolves more readily
Undercut profile
Other process steps:
 Metal coating/deposition
 Lift-off
Practical issues

Study material
Invitation to the SEM World: ”Dictionary”
Error on page 2

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Design Cad
Report
Schedule for the practical part