Top-down nanofabrication

5. Nanostructure fabrication
5.1 Top-down nanofabrication
5.2 Bottom-up nanofabrication
Philipp Altpeter
Nano 1 ─ 5.1 Top-down nanofabrication
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Nanostructure fabrication
Top-down nanofabrication
Philipp Altpeter
Nano 1 ─ 5.1 Top-down nanofabrication
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Introduction
• Motivation:
–
–
–
–
Concept of top-down fabrication processes
Materials, devices, limitations
Differences between scientific and industrial nano-fabrication
Examples
• Structure
– Part I: Basic concept, clean room architecture, lithography (ca. 90 min.)
– Part II: Deposition, etching methods and back-end fabrication
• References freely accessible
– Henderson Research Group https://sites.google.com/site/hendersonresearchgroup/helpful-primers-introductions
– Review paper: Garner C M, Lithography for enabling advances in integrated
circuits and devices. Phil. Trans. R. Soc. A. 2012; 370:4015.
– Brochures by MicroChemicals GmbH http://www.microchemicals.com/downloads/brochures.html
Philipp Altpeter
Nano 1 ─ 5.1 Top-down nanofabrication
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Terms and Definitions
•
Microsystems
– Dimensions of the functional structures in µm
– aka Micro Electro Mechanical System, MEMS
– Fluidics („lab-on-a-chip“), optics, many sensors and actuators
– Integrated Circuits (IC): DRAM, flash memory, CMOS processors
•
Nanosystem (NEMS)
– Nano-structure: two dimensions < 100 nm, e.g. electronic gates, photonic
crystals, quantum dots, apertures
– ultrathin layers < 100 nm, e.g. double-hetero or quantum-well-structures,
extremely efficient, selective mirrors, filters
•
Top-down / bottom-up
– Top-down: optimized microfabrication methods
– Bottom-up: self-organized, chemical processes (e.g. Carbon Nano Tube growth)
Philipp Altpeter
Nano 1 ─ 5.1 Top-down nanofabrication
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Industrial Chip Fabrication
Recovery of Si, wafer manufacturing; Frontend: Deposition, Lthography, Etching; Back-end:
Probing, Dicing, Bonding, Encapsuation, Burn-in and Test.
Philipp Altpeter
Process cycle for the manifacturing of semiconducting
chips (Hansch. Technik in Bayern; 04/2011.)
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Concepts and Classification
•
Patterning
– Fotolithography (reproducing)
– Charged particle beam lithography (serial wiriting)
– Nano-imprint
•
Deposition (additive methods)
– Physical or chemical
Plasma-depositions
– Evaporation, MBE, ALD
– Spin-on
– Electroplating
•
Etching (subtractive methods)
– Dry-etching (Plasma enhanced)
– Wet-etching
•
Basic concepts of top-down fabrication.
a) patterning
b) deposition OR etching
c) Resist removing OR lift-off
Back-end
– Chip- and wire bonding, Encapsulation
Philipp Altpeter
Nano 1 ─ 5.1 Top-down nanofabrication
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Clean room technology
•
•
•
What means ‚clean room‘? → avoiding and filtering of air particles
Particles with size of 1/10 of the minimal structure dimensions are critical
Possible praticle sources
– Air → circulating through filters by blowers/ventilators
– Contaminants within chemicals → clean materials (VLSI, ULSI standard)
– Abrasion from machines → grey-rooms contain infrastructure
– Contact contamination (e.g. impure tools)
– Clean room personnel – emmits 35 % of all particles!
→ „protection“ clothes, careful behavior
Movement type
Emission of particles
(dP > 0,5 µm) per minute
Regular
clothes
•
Clean room
clothes
Sitting (no movement)
3 · 105
7 · 103
Head movement
6 · 105
104
Body movement
106
3 · 104
Slow walk
3 · 106
5 · 104
Fast walk
6 · 106
105
Büttgenbach S.
Mikromechanik.
Stuttgart: Teubner; 1994.
Regulated environment (temperature, ventilation cycles, air humidity)
Philipp Altpeter
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Clean room technology
•
Air flow and filtering in clean rooms
Partikelzahl pro ft3
Partikelzahl pro m3
Ceiling
Floor
Cross-sectional drawing of an conventional clean room, air flow and ventilation.
Büttgenbach S. Mikromechanik. Stuttgart: Teubner; 1994.
Partikelgröße / µm
– DIN EN ISO 14644: ISO class n → less than 10n particles smaller 0,1 µm pro m³
– US Fed. standard 209b: US class n → less than n particles smaller 0,5 µm pro ft³
Philipp Altpeter
Nano 1 ─ 5.1 Top-down nanofabrication
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Overview lithography methods
•
reproducing methods
– shadow impact fotolithography
(smallest features ca. 1 µm)
– projection exposure (Deep-UV: 32 nm)
– X-ray lithography w/
synchrotron radiation (sub-µm,
extremly high aspect ratio,
i.e. vertical to lateral dimensions)
•
imprint, transfer printing… (soft lithography)
•
direct writing methods
– Laser Direct Imaging (sub-µm)
– electron-beam lithography (10 nm)
– Focused Ion Beam (FIB) lithography or
milling
Philipp Altpeter
templates
mask, reticle (scaling 1:4)
stamp
CAD drawing from PC
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Photolithography
•
•
•
•
Reproduction of structures in photo-sensitive films (photoresist, PR)
Light source: mercury arc lamp
– Spectral lines in regard to the absorption characteristics of the PR:
I line @ 365 nm, H line @ 405 nm, G line @ 436 nm
Photomask as template: patterned Cr film on a (UV transparent) quartz plate
Shadow impact illumination (see below), projection printing
Principle of shadow impact lithography. Madou M. Fundamentals
of Microfabrications. Boca Raton: CRC Press; 1997.
Philipp Altpeter
UV part of the Hg spectrum.
Koch Ch, Rinke T J. Lithography. Ulm: MicroChemicals; 2006.
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Photolithography
Maskaligner Karl Suss MJB 3
Philipp Altpeter
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Photoresist technology
•
Positive PR (solubility of exposed areas increased)
– Composition: Novolak resin, photo-active compound (DNQ), solvents
•
Negative PR (solubility decreased)
– Crosslinking or polymerization (chain growth) induced by proper irradiation
– Slightly reduced resolution through diffusion of developer and swelling of
structures
– Chemical amplification: photon generates acid catalyst which initiates
depolymerization; quencher added to quench the diffusion of the catalyst
After
Exposure
After
Development
Alteration of solubility of a CA positive-tone resist
over the course of the process.
Philipp Altpeter
Profile characteristics of positive-tone PR (a) and negative-tone PR (b).
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Typical process flow
•
Pretreatment, adhesion promotion
Surface chemistry: hydrophilic → hydrophobical (non-polar)
by means of: using a primer, heating, oxygen-plasma
•
Spin coating (defines the thickness of the
PR layer considering the viscosity)
•
Softbake (evaporating solvents)
•
Exposure (photo-chemical activation)
•
Post-Exposure-Bake, PEB (increasing crosslinking; finishing the photo-reaction)
•
Development (Dissolution either of exposed or non-exposed areas)
•
Hardbake (better adhesion and chemical-mechanical stability)
•
De-scumming (Etching of resist residues in O2-plasma)
Philipp Altpeter
Principle of resist coating by spinning.
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Basic chemistry of mid-UV, positive-tone PR
DNQ (unexposed)
dissolution inhibitor
h·v
+H2O
DNQ (exposed)
dissolution promotor
Philipp Altpeter
Chemical structure of a conventional positive resist.
Koch Ch, Rinke T J. Lithography. Ulm: MicroChemicals; 2006.
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Physical resolution limits
•
Resolution: min. resolvable distance between two points (see Rayleigh criterion).
bmin ≈ 1,5 ⋅ λ ⋅ (g + t R 2)
•
bmin: minimal feature size
g: proximity gap
tR: resist thickness
Light diffraction, proximity illumination
– contact mode
– layer thickness
– edge bead
– particles, bubbles
Proximity distance caused by edge bead of the coated resist film.
Intensity distribution depending on wavelength, slit width, gap
Koch Ch, Rinke T J. Lithography. Ulm: MicroChemicals; 2006.
Philipp Altpeter
Diffraction pattern by a slit aperture
from near field to far field,
a) Fraunhofer diffraction,
b) Fresnel diffraction.
Tipler P A. Physik.
Heidelberg, Berlin: Spektrum; 1994.
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Consequences of light diffraction
SEM micrograph: diffraction pattern reproduced in a photo resist film after development;
Slit width in photomask: 4 µm.
Philipp Altpeter
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Resist-based resolution limits
•
Contrast and sensitivity (diffraction → grey areas)
Low contrast: resist edges shallow, poor defined
D0
T(D) = T 0 ⋅ γ ⋅ ln
D
T(D): residual resist after
development
Contrast curve of a standard positive resist;
Consider logarithmical abscissa.

D0 
1
γ=
= log

log D0 − log D1 
D1 
•
•
−1
γ: contrast
Do: exposure dose
D1: threshold dose
Smallest achievable structure width by means of shadow impact litho: ≈ 1 µm
for DUV and below: high efficient, chemical amplified (CA) resists required
Philipp Altpeter
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Resist optimization
•
Quality of a high-end CA-photoresist given by: Z = R³ x LER² x S
R: ultimate Resolution
LER: Line Edge Roughness
S: Sensitivity → throughput
•
Chemical amplified (CA) resists limited by the
‚Triangle of death‘
Sensitivity
Acid diffusion length
Line Width
Pitch
Resolution vs. Line Edge Roughness.
•
half-pitch = max. Resist thickness
Philipp Altpeter
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Projection exposure
•
•
Structures from photo mask will be
demagnified and printed through a
reduction lense
Wafer stepper moves wafer from each
position to the next where the illumination
takes place
bmin ≈ k 1 ⋅ λ
NA
bmin: minimal feature size
k1: prefactor, ca. 0.6 in case
of incoherent light
NA: Numerical Apertur of the
lense system
•
•
•
DOF ≈ k 2 ⋅ λ
NA 2
k2: prefactor, ca. 0.5
Depth of Focus: consider resist thickness
and complex topologies!
Advantages: better resolution, more
different structures (layer levels)
on one photomask
Disadvantages: expensive,
Schematic of simplified step-and- repeat projection exposure tool system.
http://henderson.chbe.gatech.edu/Introductions/microlithography%20intro.htm
time consuming exposure
Philipp Altpeter
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Lithography evolution
•
•
•
•
high intensity and low wavelength light sources required for high resolution
ArF excimer laser (193 nm, DUV) and immersion litho (increase of NA up to 1.3)
optical transitions in highly elaborated plasma sources for EUV (13.5 nm): bmin<20nm
very expensive! Ultra-high vacuum condition, multi-layer mirrors instead of lenses to
avoid light absorption and complicated mask technology (phase masks,
compensation structures)
Ultra-modern lithography tool of ASML working at Extreme UV (pricing > 60 M€).
Hansch. Technik in Bayern; 04/2011.
Philipp Altpeter
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Laser Direct Imaging
•
•
Laser Direct Imaging (LDI): monochromatic, coherent, gaussian Laser beam (375
or 405 nm) is scanning across the surface (Vector-scan)
Beam deflection by means of an
acousto-optical deflector (AOD)
in combination with motorized stage
Software
Transducer generates acoustic wave
through the quartz crystal
Refrective index n alters periodically
in order of the density variation
Bragg diffraction
→ Bragg angle proportional to f
of the acoustic wave (Θ ~ ∆f)
Intensity depends on RF power
of the transducers.
•
•
Typical specs:
Spot size = 1…3 µm
Beam positioning in nm range
Rayleigh length 2…20 µm
Control via PC, design defined by CAD
Philipp Altpeter
Beam path of a Laser Direct Imaging system.
By courtesy of Anze Jeric (LPKF).
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Standing wave issues
•
Advantages: maskless, no diffraction on photomask structures, usually better resolution
achievable
flexible, Design quickly and easily adaptable
Dose variation / dose test doable with one exposure
•
Disadvantages: Riffled edges (see below) caused by standing waves
reduced resolution
Large areas need a long over-all exposure time
Stitching error at extended structures
(e.g. wave guides or channels)
∆z = (λ / 4) n-1
Riffled sidewalls (top) caused by illumination with monochromatic light. Model of standing waves
within photo resist (right). Madou M. Microfabrication. Boca Raton: CRC Press; 1997.
Philipp Altpeter
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Electron beam lithography
•
Requirements: Scanning Electron Microscope (SEM) with pattern generator, electrostatic beam blanker and a PC to control the system
Abbe resolution criterion:
d min = k 1 ⋅
Beam path of a SEM with Thermal-Field-Emitting
(TFE) cathode.
Manual LEO-FE-REM 982.
λ
n ⋅ sin α
dmin: min. dimension
n · sin α: Numerical Aperture (NA)
λ: wavelength
Free, accelerated electrons
→ very small De Broglie wavelength
e.g. at 10 keV << 1 nm!
Comparison: Hg transition at 365 nm (I line),
1. Kathode
and even EUV at 13.5 nm
2. Suppressor electrode (housing)
→ charged particles (like electrons)
excellently focusable and deflectable!
Philipp Altpeter
4. Extractor electrode
5. Anode
7. Condensor lense
10., 11. Objective lense
12. Sample
Leaving area and amount of
Secondary Electrons (SE) depend on
topography (edge effect).
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Electron beam lithography
•
•
•
•
•
•
Beam creation with Primary
Electrons (PE)
Focusing of the PE beam by
means of electrons optics
(electromagnets and
aperturs)
Scanning across the sample
surface via deflection coils
Emission of Secondary
Electrons (SE) at the e-beam
spot
Amount of SE depends on
topography (→ edge effect)
Detector „soaks in“ SE and
produces an intensity signal
depending on the surface
profile
How imaging of scanning electron microscopy works.
Hawkes P W. Scanning Electron Microscopy. Heidelberg: Springer; 1985.
Philipp Altpeter
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Focusing of charged particle beams
•
•
•
inhomogenous B field along pole shoes of electromagnets serves as a convex
lense for electrons (or ions)
If electrons (coming from the primary beam) have a component of the velocity vector
perpendicular to the B field v┴ , they sense the
Lorentz force F = - e (v × B)
Focus adjustable by variation of the field strength: f ~ B02
Aberrations of optical systems.
Hawkes P W. Scanning Electron
Microscopy. Heidelberg: Springer; 1985.
Schematic of an electron lense.
Hawkes P W. Scanning Electron
Microscopy. Heidelberg: Springer; 1985.
•
Typical aberrations
– Spherical aberration (Öffnungsfehler)
– Chromatic aberration (Farbfehler)
– Astigmatism, cylinder asymetries
– Diffraction error
Philipp Altpeter
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E-beam lithography: exposure methods
Fixed Beam Moving Stage
• Software turns design
into polygons
• Pattern generator scans
these polygons one after
another and from pixel to
pixel
• Step size (between
pixels) depends on write
field size and address
range of the pattern
generator
• excellent resolution, but
stitching errors at
extended structures,
low throughput,
low-cost
Philipp Altpeter
• Software turns design into
polygons
• smart alignment of beam and
aperture to expose the whole
area of one polygon
• Beam stationary, stage is driving
• Trace of the stage is defined by
vectors and stage coordinates
• high throughput, but
expensive hardware
• for high resolution, interferometer required; NO stitching
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E-beam lithography: vector scan method
D = J ⋅ t Dwell
E-gun
Anode
Alignment coils
D: dose; typ. 100 µC · cm-2 → resist properties,
developer temperature, substrate
J: probe current density → cathode, acceleration
voltage, aperture diameter
tDwell: dwell time → data bus frequency, beamblanker, deflection coils resp. scan electronics
Condenser lens
Aperture
Beam blanker
electron beam
Pattern
generator
50 pA
Objective lens
200
µm
Scan coils
Write field size
and step size
→ adress range, e.g.
16 Bit DAC (216 = 65536),
WF: 100 µm
→ Step size: 1.5 nm
3 nm
Vector scan patterning with a conventional SEM.
Trainingsunterlagen Fa. Raith.
Philipp Altpeter
Matching of microscope
magnification and write field
size
Definition of write field.
Trainingsunterlagen Fa. Raith.
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EBL vector scan: stitching errors
Stage movement
WF1
Shift in X (ca. 200 nm)
Gap due to Zoom error (0.5 %)
Write Field Border
WF2
Philipp Altpeter
WF Edge
Length
100 µm
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Fixed beam moving stage
Laser interferometric stage.
Trainingsunterlagen Fa. Raith
Precise positioning by means of an
interferometer.
Hoffmann J. Taschenbuch der
Messtechnik. München, Wien: Carl Hanser;
2002.
Feedback
Philipp Altpeter
FBMS: travelling stage, ‚resting‘
beam; feedback from interferometer
to beam deflection.
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Shaped beam lithography
•
•
•
beam shape defined by a system of apertures instead of
scanning pixel by pixel
‚large‘ areas are quickly exposed
specialized, complicated system and expensive!
Philipp Altpeter
Shaped e-beam lithography by
Leica Microsystems Lithography
GmbH
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Typical SEM with e-beam attachments
LEO FE-REM GEMINI 982
Philipp Altpeter
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Resolution limits of electron beam lithography
•
Theoretical limit given by the De Broglie wavelength of the PE
•
But: backscattered (BSE) and
secondary electrons (SE) cause
an exposure too
→ proximity effect
BSE: E > 50 eV
SE: E = 2 … 20 eV
Tread depth of SE,
around 3 nm
•
aberrations (s. slide #25),
coulomb repulsion
•
Resist properties (contrast)
most important: PMMA (positive;
chain scission mechanism),
PMMA
HSQ, SU-8… (negative-tone) Silizium
•
interaction zone → atomic number
and acceleration voltage
Philipp Altpeter
Beam diameter d shrinks with higher acceleration voltage
d
10 kV
d
d
20 kV
Scattering events
simulated by Monte
Carlo in PMMA and Si
depending on
acceleration voltage.
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Cathodes / Electron emitters
•
Emitter types:
– thermal emission (Heizkathode)
Requirements: low work function, thermally stable → W
optimal material: ceramic LaB6 (lower work function than W, very small emission
area)
– (cold) field emission: E-field causes tunneling of electrons
Advantage: smallest, atomic emission area → almost no chromatic aberrations
and an excellent, small spot
Disadvantage: unstable emission current, extremly high vacuum required
– best of both: thermal field emission (TFE of Schottky cathode). Tip is covered
with Zircon Oxide (W/ZrO) to reduce work function
SEM images of an TFE cathode. Left side: tip surrounded by the suppressor electrode.
(Mag. 200x); Center: tip diameter ≈ 300 nm; Right: foto of the whole device.
Philipp Altpeter
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Enhancements of e-beam lithography
•
Focus electron beam induced processing
Using precursor gases for direct etching or
deposit of materials
Left side: Principle of electron beam
induced deposition (EBD). Center:
Image of the injection needles.
Right: SEM micrograph of the
needle at 20x magn.
Trainingsunterlagen u. Handbuch
der Fa. Raith.
Nanomanipulator. Right side:
Drawing of the complete device
(Handbuch Fa. Raith); left: SEM
image shows a nanomanipulator
bending a GaAs pillar (ca. 30.000x
magn.), ∅ tip ≈ 300 nm,
∅ pillar ≈ 1 µm.
•
Nanomanipulator
– moves free-standing nano-structures mechanically
– electrical probing of nano-devices
– Smallest step size < 4 nm
Philipp Altpeter
Nano 1 ─ 5.1 Top-down nanofabrication
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Nanoimprint Lithography (NIL)
•
Substrate surface coated with a polymer (a)
•
Imprint template/stamp pushed into polymer (b)
•
•
Polymer cures thermally or by UV radiation
Remove of imprint tool, polymer image remains (c)
•
Critical challanges: low-defect template; polymer:
good adhesion to wafer, no adhesion to imprint tool
Principle of nano transfer printing.
Fakhr O, Altpeter P, Karrai K, Lugli P. Easy Fabrication of Electrically
Insulating Nanogaps by Transfer Printing. Small 7 2533 (2011)
Philipp Altpeter
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Example1: Mix & Match
•
•
•
•
Substrate: Silicon-On-Insulator (SOI); device based on Metal-Oxide-Silicon (MOS)
Field-Effect-Transistor (FET)
Mix and match: combination of both, fotolithography and EBL
quasi-metallic structure through extensive doping of source and drain (Silicon)
thermal oxidation of Si and wet etching of SiO2: thinning!
Wiring and bond pads
hair
Contact printing lithography
pattern
SEM image of a Silicon nanowire.
Tilke A. Dissertation. München; 2000.
e-beam pattern
Philipp Altpeter
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Sense of the magnitude…
Philipp Altpeter
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Example 2: laterally defined Double Quantum Dot (DQD)
Chip carrier and
bond wires
(‚spider legs‘)
QD
QD
(a)
20x magn.
Nano magnet (Co), 2nd EBL step
electric gates (Au), 1st EBL step
(b)
50000x magnified
SEM images of a quantum dot structure; (a) sample attached to carrier,
(b) Co nano magnets, (c) Au gates in the center.
Substrate: molecular beam epitaxy (MBE) grown heterostructure GaAs/AlGaAs
Kupidura D. Dissertation. München 2009.
(c)
2000x magn.
Philipp Altpeter
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Example 3: suspended quantum dot structure
•
Additional etching steps required: anisotropic Reactive Ion Etching and isotropic
sacrificial layer wet etching.
Source
Drain
G1
G2
SEM images of a suspended double quantum dot (DQD) structure. Left image: top view. Right image: tilted at 85°
Below: Process flow: starting from the heterostructure, followed by several lithography steps, suspended wire via sacrificial layer etching in the end.
Rössler C. Dissertation. München 2008.
Process flow
Philipp Altpeter
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Example 4: high reflective Bragg mirror
•
•
•
•
•
1-dimensional nano-structure (stack of layer)
quarter wave Bragg reflector
alternating double layer of GaAs (high index, 3.480) and Al0.92Ga0.08As (low index,
2.977, @ 1.064 um) or other material combinations like SiO2 / Ta2O5
deposited by Molecular Beam Epitaxy (MBE) or (epitaxial) Sputtering
Interference of λ/4 double layers
s. below.
Top: principle of a Bragg reflector (batop.de/information/r_Bragg.htm)
Right figure: schematic of an EBD grown reflector, cross-sectional
SEM image and reflectance (meausered: ret dots; simulated: blue line)
Cole et al. Nature Photonics 7, 644–650 (2013)
Philipp Altpeter
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Example 5: photonic crystals
•
•
•
•
•
Photonic crystal: periodic, alternating
structure of the refractive index
Compared to the electronic nature of
semiconductors → band structure etc.
High-resolution EBL, perfectly shaped
lattice elements needed
anisotropic Silicon etching with a
special EBL resist (ZEP)
as soft-mask
Shape and line edge roughness
strongly defines the quality of these
structures
Different optical devices – such as: waveguides, beam splitter, resonators –
fabricated by EBL (below); SEM image of a 1-dimensional waveguide with a
small defined defect in the center (top left); simulated and measured data of
the light transmission through this structure,w hich behaves like a filter
(top right)
Birner et al. Phys. Blätter 55 (1999).
Philipp Altpeter
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Example 6: Electron Beam Deposition AFM tips
•
•
•
•
Focussed Electron Beam Induced Deposition (FEBID)
on the apex of a conventional AFM pyramide
carbonic precursor decomposed by focussed electron beam
into non-volatile and volatile products
sharp pillar made of amorphous,
diamond-like Carbon: strong and steady
Application: e.g. measurement of roughness in deep trenches
Down left: principle of Atomic Force Microscopy (AFM); from: the
NanoWizard AFM Handbook, Fa. JPK, v1.3, 08/2005.
SEM images of atomic EBD tips (right), courtesey of nanotools GmbH.
ultra-sharp probes for
atomic force microscopy
length: up to 6,000 nm
radius: 5 nm
optical lever
Philipp Altpeter
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