5.1 top-down nanofabrication 5.2 bottom-up …...5.1 top-down nanofabrication 5.2 bottom-up...

5. Nanostructure fabrication

5.1 Top-down nanofabrication

5. Nanostructure fabrication

5.2 Bottom-up nanofabrication

1Nano 1 ─ 5.1 Top-down nanofabricationPhilipp Altpeter

5.2 Bottom-up nanofabrication

Nanostructure fabrication

Top-down nanofabrication

Nanostructure fabrication


Introduction

• Motivation:– Concept of top-down fabrication processes– Materials, devices, limitations– Differences between scientific and industrial nano-fabrication– Examples

• Agenda– Part I: Basic concept, clean room architecture, lithography (ca. 90 min.)– Part II: Deposition, etching methods and back-end fabrication– Cleanroom tour on Thursday (two groups, 45 min. each)


• 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 integratedcircuits and devices. Phil. Trans. R. Soc. A. 2012; 370:4015.

– Brochures by MicroChemicals GmbH http://www.microchemicals.com/downloads/brochures.html

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 vs. bottom-up– Top-down: optimized microfabrication methods– Bottom-up: self-organized, chemical processes (e.g. Carbon Nano Tube growth)

Industrial Chip Fabrication


Process cycle for the manifacturing of semiconducti ng chips (Hansch. Technik in Bayern; 04/2011.)

Recovery of Si, wafer manufacturing; Frontend : Deposition, Lthography, Etching; Back-end : Probing, Dicing, Bonding, Encapsuation, Burn-in and Test.

Concepts and Classification

• Patterning (define patterns)– 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

• Back-end– Chip- and wire bonding, Encapsulation

Basic concepts of top-down fabrication includingpatterning, deposition or etching and resist remov al,

Resulting structures are either lift-off or etched structures or trenches

Clean room technology

• What means ‚clean room‘? → avoiding and filtering of air particles• Possible particle sources

– Air → circulating through filters by blowers– Contaminants within chemicals →

clean materials (VLSI, ULSI standard)– Abrasion from machines grey-rooms – Abrasion from machines → grey-rooms

contain infrastructure– Contact contamination (e.g. impure tools)– Clean room personnel – emmits 35 % of

all particles! → „protection“ clothes, appropriate behavior!

Movement type Emission of particles(dP > 0,5 µm) per minute

Regular Clean room


• Regulated environment (temperature, ventilation cycles, air humidity)

Regular clothes

Clean roomclothes

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.

Clean room technology

• Air flow and filtering in clean rooms

Ceiling

Num

ber

of p

artic

les

per

m3 N

umber of particles per ft 3

ISO class 5US class 100

ISO class 4US class 10ISO class 4

US class 10 ISO class 6US class 1000

ISO class 6US class 1000


– 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

Particle size / µmCross-sectional drawing of an conventional clean ro om, air flow and ventilation. Büttgenbach S. Mikromechanik. Stuttgart: Teubner; 1 994.

Floor

Overview lithography methods

• reproducing methods– shadow impact fotolithography

(smallest features ca. 1 µm)– projection exposure (Deep-UV: 32 nm) mask, reticle (scaling >1)

templates

– 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

mask, reticle (scaling >1)

stamp, mold


• direct writing methods– Laser Direct Imaging (sub-µm)– electron-beam lithography (10 nm)– Focused Ion Beam (FIB) lithography or

milling

CAD drawing from PC

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 (opaque) Cr film on a (transparent) quartz plate• Photomask as template: patterned (opaque) Cr film on a (transparent) quartz plate• Shadow impact illumination (see below), projection printing


UV part of the Hg spectrum. Koch Ch, Rinke T J. Lithography. Ulm: MicroChemical s; 2006.

Principle of shadow impact lithography. Madou M. Fu ndamentalsof Microfabrications. Boca Raton: CRC Press; 1997.

FotomaskSample

Photolithography


Maskaligner Karl Suss MJB 3

Photoresist technology

• Positive PR (solubility of exposed areas increased)– Typical composition: Novolak resin, photo-active compound (PAC), solvents– PAC acts as a dissolution promotor

• Negative PR (solubility decreased)– Crosslinking or polymerization (chain growth) induced by proper irradiation– 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


Profile characteristics of positive-tone PR (a) a nd negative-tone PR (b).

After Exposure

After Development

Solubility of positive-tone photoresist in terms of composition and after exposure. Dammel R R; SPIE 1993.

Typical process flow

• Pretreatment, adhesion promotionSurface chemistry: hydrophilic → hydrophobical (non-polar)by means of: using a primer, heating, oxygen-plasma

• Spin coating (defines the thickness of the Dispenser

PR layer considering the viscosity)

• Softbake (evaporating solvents)

• Exposure (photo-chemical activation)

• Post-Exposure-Bake, PEB (increasing crosslinking; finishing the photo-reaction)

Principle of resist coating by spinning.

Vacuum


• 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)

Basic chemistry of mid-UV, positive-tone PR

DNQ (unexposed) dissolution inhibitor


Chemical structure of a conventional positive resis t.Koch Ch, Rinke T J. Lithography. Ulm: MicroChemical s; 2006.

dissolution inhibitor

DNQ (exposed) dissolution promotor

h·v+H2O

More advanced: chemical amplification (CA)


Principle of chemical aplification in terms of phot oresist technology.Allen R D et al. Microlithography, Micromachining a nd Microfabrication. Washington: SPIE Press; 1997.

Physical resolution limits

• Resolution: min. resolvable distance between two points (see Rayleigh criterion).

• Light diffraction , proximity illumination, enhanced by

( )25,1min Rtgb +⋅⋅≈ λbmin: minimal feature sizeg: proximity gaptR: resist thickness

– contact mode– layer thickness– edge bead– particles, bubbles

Proximity distance caused by edge bead of the coate d resist film.

Incr

asin

g di

stan

ce: a

pert

ure �

scre

en


Diffraction pattern by a slit aperture from far field to fnear field, a) Fraunhofer diffraction, d) Fresnel diffraction. Tipler P A. Physik. Heidelberg, Berlin: Spektrum; 1994.

Intensity distribution depending on wavelength, sli t width, gapKoch Ch, Rinke T J. Lithography. Ulm: MicroChemical s; 2006.

Incr

asin

g di

stan

ce: a

pert

ure

Consequences of light diffraction


SEM micrograph: diffraction pattern reproduced in a photo resist film after development;Slit width in photomask: 4 µm.

Resist-based resolution limits

• Contrast and sensitivity (diffraction → grey areas)Low contrast: resist edges shallow, poor defined

D

DTT(D) 0

0 ln ⋅⋅= γ

11

−

== Dγγ: contrast (slope of contrast curve)

Contrast curve of a DNQ based positive resist;Consider logarithmical abscissa. Dammel R R; SPIE 1993.

DT(D): residual resist after

development


• Smallest achievable structure width by means of shadow impact litho: ≈ 1 µm• for DUV and below: high efficient, chemical amplified (CA) resists required to combat

the larger absorption at smaller wavelengths

1

0

10

logloglog

1

=

−=

DD

D D

γγ: contrast (slope of contrast curve)Do: exposure doseD1: threshold dose

Resist optimization

• Quality of a high-end CA-photoresist given by: Z = R³ x LER² x SR: ultimate ResolutionLER: Line Edge RoughnessS: Sensitivity → throughput

• Chemical amplified (CA) resists limited by the‚Triangle of death‘

SensitivityLine Width


• half-pitch = max. Resist thickness

Acid diffusion length

Resolution vs. Line Edge Roughness.

Pitch

• Structures from photo mask will bedemagnified and printed through areduction lense

• Wafer stepper moves wafer from eachposition to the next where the illuminationtakes place

Projection exposure

takes place

• Depth of Focus: consider resist thickness

NAkb λ⋅≈ 1min

bmin: minimal feature sizek1: prefactor, ca. 0.6 in case

of incoherent lightNA: Numerical Apertur of the

lense system

22 NAkDOF λ⋅≈

k2: prefactor, ca. 0.5


• Depth of Focus: consider resist thicknessand complex topologies!

• Advantages: better resolution, moredifferent structures (layer levels) on one photomask, no contamination

• Disadvantages: expensive, time consuming exposure

Schematic of simplified step-and- repeat projection exposure tool system.http://henderson.chbe.gatech.edu/Introductions/micr olithography%20intro.htm

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, avoid light absorption and complicated mask technology (phase masks, compensation structures)


Ultra-modern lithography tool of ASML working at Ex treme UV (pricing > 60 M€).Hansch. Technik in Bayern; 04/2011.

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 wavethrough the quartz crystalRefrective index n alters periodicallyin order of the density variationBragg diffraction→ Bragg angle proportional to fof the acoustic wave (Θ ~ ∆f)Intensity depends on RF powerof the transducers.


• Typical specs: Spot size = 1…3 µmBeam positioning in nm rangeRayleigh length 2…20 µm

• Control via PC, design defined by CAD

Beam path of a Laser Direct Imaging system.By courtesy of Anze Jeric (LPKF).

Standing wave issues

• Advantages : maskless, no diffraction on photomask structures, usually better resolutionachievableflexible, Design quickly and easily adaptableDose variation / dose test doable with one exposure

• Disadvantages : Riffled edges (see below) caused by standing wavesreduced resolutionreduced resolutionLarge areas need a long over-all exposure timeStitching 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. Microfabricat ion. Boca Raton: CRC Press; 1997.

Electron beam lithography

• Requirements: Scanning Electron Microscope (SEM) with pattern generator, electro-static beam blanker and a PC to control the system

Beam path of a SEM with Thermal-Field-Emitting (TFE) cathode.Manual LEO-FE-REM 982.

αλsin

1min ⋅⋅=n

kd

Abbe resolution criterion:

αsin1min ⋅n

dmin: min. dimensionn · sin α: Numerical Aperture (NA)λ: wavelength

Free, accelerated electrons→ very small De Broglie wavelength


1. Kathode2. Suppressor electrode (housing)4. Extractor electrode5. Anode7. Condensor lense10., 11. Objective lense12. Sample

Leaving area and amount of Secondary Electrons (SE) depend on topography (edge effect).

e.g. at 10 keV << 1 nm!Comparison: Hg transition at 365 nm (I line),and even EUV at 13.5 nm

→ charged particles (like electrons) excellently focusable and deflectable!

Electron beam lithography

• Beam creation with Primary Electrons (PE)

• Focusing of the PE beam by means of electrons optics (electromagnets and (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)


How imaging of scanning electron microscopy works. Hawkes P W. Scanning Electron Microscopy. Heidelber g: Springer; 1985.

topography (→ edge effect)• Detector „soaks in“ SE and

produces an intensity signal depending on the surface profile

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• Focus adjustable by variation of the field strength: f ~ B0

Schematic of an electron lense. Hawkes P W. Scanning Electron Microscopy. Heidelberg: Springer; 1985.

Aberrations of optical systems. Hawkes P W. Scanning Electron

Microscopy. Heidelberg: Springer; 1985.


• Typical aberrations– Spherical aberration (Öffnungsfehler)– Chromatic aberration (Farbfehler)– Astigmatism, cylinder asymetries– Diffraction error

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

• Beam stationary, stage is driving

• Software turns design into polygons

• smart alignment of beam and aperture to expose the whole area of one polygon

• high throughput, but


field size and address range of the pattern generator• excellent resolution, but stitching errors at extended structures, low throughput,low-cost

• Trace of the stage is defined by vectors and stage coordinates

• for high resolution, inter-ferometer required; NO stitching

• high throughput, but expensive hardware

Anode

E-gun

E-beam lithography: vector scan method

DwelltJD ⋅=D: dose ; typ. 100 µC · cm-2 → resist properties, developer temperature, substrate

J: probe current density → cathode, acceleration voltage, aperture diameter

Alignment coils

Condenser lens

Scan coils

Aperture

Objective lens m

50 pA

electron beam

voltage, aperture diameter

tDwell: dwell time → data bus frequency, beam-blanker, deflection coils resp. scan electronics

Beam blanker

Write field size

Pattern generator


Objective lens

200

µm

3 nmVector scan patterning with a conventional SEM.

Trainingsunterlagen Fa. Raith.Definition of write field.Trainingsunterlagen Fa. Raith.

Write field sizeand step size → adress range, e.g.16 Bit DAC (216 = 65536),WF: 100 µm → Step size: 1.5 nm

Matching of microscope magnification and write field size

EBL vector scan: stitching errors

WF1Stage movement

Shift in X (ca. 200 nm)

Write Field Border

WF Edge

Gap due to Zoom error (0.5 %)


WF2WF Edge Length100 µm

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.


FBMS: travelling stage, ‚resting‘ beam; feedback from interferometer to beam deflection.

Feedback

Shaped beam lithography


• beam shape defined by a system of apertures instead ofscanning pixel by pixel

• ‚large‘ areas are quickly exposed• specialized, complicated system and expensive!

Shaped e-beam lithography by Leica Microsystems Lithography GmbH

Typical SEM with e-beam attachments


LEO FE-REM GEMINI 982

Resolution limits of electron beam lithography

• Theoretical limit given by the De Broglie wavelength of the PE

• But: backscattered (BSE) andsecondary electrons (SE) causean exposure too → proximity effect

Beam diameter d shrinks with higher acceleration voltage

→ proximity effect

BSE: E > 50 eVSE: E = 2 … 20 eV

• aberrations (s. slide #26), coulomb repulsion

• Resist properties (contrast)

Escape depth of SE, around 3 nm

d dd

Pen

etra

tion

dept

h


• Resist properties (contrast) most important: PMMA (positive; chain scission mechanism), HSQ, SU-8… (negative-tone)

• interaction zone → atomic numberand acceleration voltage

Scattering events of PE simulated by Monte Carlo in PMMA and Si depending on acceleration voltage.

10 kV 20 kV

VacuumPMMA

Silicon

Pen

etra

tion

dept

h

Distance from beam center / µm

Cathodes / Electron emitters

• Emitter types:– thermal emission (Heizkathode)

Requirements: low work function, thermally stable → Woptimal material: ceramic LaB6 (lower work function than W, very small emission area)

– (cold) field emission: E-field causes tunneling of electronsAdvantage: smallest, atomic emission area → almost no chromatic aberrations and an excellent, small spotDisadvantage: 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 surrou nded by the suppressor electrode. (Mag. 200x); Center: tip diameter ≈ 300 nm; Right: foto of the whole device.

Some enhancements of e-beam lithography

• Focus electron beam induced processingUsing 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.

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.

Trainingsunterlagen u. Handbuch der Fa. Raith.


• Nanomanipulator– moves free-standing nano-structures mechanically– electrical probing of nano-devices– Smallest step size < 4 nm

∅ pillar ≈ 1 µm.

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)


• 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 Fabric ation of Electrically

Insulating Nanogaps by Transfer Printing. Small 7 2 533 (2011)

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!

hair

Contact printing lithography

Wiring and bond pads


Contact printing lithographypattern

e-beam pattern

SEM image of a Silicon nanowire.Tilke A. Dissertation. München; 2000.

Sense of the magnitude…


Example 2: laterally defined Double Quantum Dot (DQD)

Chip carrier and bond wires (‚spider legs‘)

QD

(a)20x magn.

Nano magnet (Co), 2nd EBL step

QD

electric gates (Au), 1st EBL step

QD


(b)

(c) 2,000x magn.

50,000x magnified

SEM images of a quantum dot structure; (a) sample a ttached to carrier, (b) Co nano magnets, (c) Au gates in the center. Substrate: molecular beam epitaxy (MBE) grown heter ostructure GaAs/AlGaAsKupidura D. Dissertation. München 2009.

electric gates (Au), 1st EBL step

• Additional etching steps required: anisotropic Reactive Ion Etching and isotropic sacrificial layer wet etching .

Example 3: suspended quantum dot structure

Source Drai n

G1 G2

SEM images of a suspended double quantum dot (DQD) structure. Left image: top view. Right image: tilte d at 85°Below: Process flow: starting from the heterostruct ure, followed by several lithography steps, suspend ed wire via sacrificial layer etching in the end.Rössler C. Dissertation. München 2008.


Rössler C. Dissertation. München 2008.

Process flow

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) Sputter ing• deposited by Molecular Beam Epitaxy (MBE) or (epitaxial) Sputter ing• Interference of λ/4 double layers

s. below.


Top: principle of a Bragg reflector (batop.de/infor mation/r_Bragg.htm) Right figure: schematic of an EBD grown reflector, cross-sectional SEM image and reflectance (meausered: ret dots; sim ulated: blue line)Cole et al. Nature Photonics 7, 644–650 (2013)

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 shapedlattice elements neededlattice elements needed

• anisotropic Silicon etching with aspecial EBL resist (ZEP) as soft-mask

• Shape and line edge roughnessstrongly defines the quality of these structures


Different optical devices – such as: waveguides, bea m splitter, resonators –fabricated by EBL (below); SEM image of a 1-dimensi onal waveguide with a small defined defect in the center (top left); simu lated and measured data ofthe light transmission through this structure,w hic h behaves like a filter (top right)Birner et al. Phys. Blätter 55 (1999).

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 beaminto non-volatile and volatile products

• sharp pillar made of amorphous, diamond-like Carbon: strong and steadydiamond-like Carbon: strong and steady

• Application: e.g. measurement of roughness in deep trenches

ultra-sharp probes for atomic force microscopy

length: up to 6,000 nmradius: 5 nm

Down left: principle of Atomic Force Microscopy (AF M); from: the NanoWizard AFM Handbook, Fa. JPK, v1.3, 08/2005. SEM images of atomic EBD tips (right), courtesey of nanotools GmbH.


optical lever

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