TOOLS OF MICRO & NANO

FABRICATION

OPTICAL LITHOGRAPHY

• Lithography Process of transferring

patterns to semiconductor materials in analogy

to photographic process

Optical lithography……Light sensitive resist

expose to UV light

Exposed region becomes more soluble and the pattern is reproduced on the resist layer

Silicon Wafer

Underlying semiconductor material

etched by acid

Solvents remove resist

Optical Lithography

• Photoresistive resine

• Patterns: Masks

• Wavelenght resolution dependant

Resolution Limits• Contact

Advantages:

●Good resolution

Drawbacks:

●Masks thin and flexible●Use ->defects

Resolution Limits• Proximity

Advantages:

●Masks lifetime high

Drawbacks:

●Resolution not as good●Diffraction●Fresnel diffraction

Gap~ 20-50 μm

Resolution Limits• Projection

Advantages:

●Good resolution●No deterioration●Image smaller than mask

Drawbacks:

●Fraunhoffer diffraction●Compromise between resolution and depth of focus

• The resolution of the patterning process is determined by

wavelength of the radiation used

• Maximum feature size ( resolution)

= C x Wavelength / numerical aperture

Where,

C is material dependent constant

Hence higher the energy more the resolution will be

RESOLUTION

Higher resolution lithography

• X-Ray – 1-1.5nm range lack of refractive

x-ray optics

• Extreme UV Lithography 10-14nm

wavelength

Extreme Ultraviolet Lithography

• Small wavelengthBetter resolution

• No lenses: mirrors

• Laser plasma sources

• 10 nm

Nanoimprint

• techniques:

Heat resineCool down

UV radiations

EUV soon in fabrication

NanoimprintE beamfor 22nm

X Rays difficult

Patterning Techniques

The electron beam lithography• Types of EBL

1. Electron Beam Direct Write2. Electron Projection Lithography

Bragg-Fresnel lens for x-rays Paul Scherrer Institute

Electron Beam Direct Write• An electron gun or electron

source that supplies the electrons.

• An electron column that 'shapes' and focuses the electron beam.

• A mechanical stage that positions the wafer under the electron beam.

• A wafer handling system that automatically feeds wafers to the system and unloads them after processing.

• A computer system that controls the equipment.

Electron Beam Direct WriteTypes of electron guns• Thermoionic• Field emission

Write-field (WF)

Scanning methods• Raster scan• Vector scan

Raith 150 Manual (Nanostructure Physics Dept. KTH) Anders Liljeborg

Specifications, a real example

Raith150• Beam size ≤ 2nm @ 20

keV• Beam energy 100eV - 30

keV • Minimum line width 20 nm • Import file format GDSII,

DXF, CIF, ASCII, BMP

Electron Projection LithographyElectron Beam Direct

Write

• SCALPEL (Bell Laboratories and Lucent technologies) 1995

• PREVAIL (IBM) 1999

Limited throughput

Electron Projection Lithography

Huge penetration depth of electrons

New solutions

Electron Projection Lithography

• SCALPEL– High contrast– Image reduction

• PREVAIL– Larger effective field

Electron beam resists

1. Important parameters 2. Types of resist3. Resist limitations

EBL resists

Types of resist• Positive resist Polymethyl

methacrylate (PMMA) • Negative resist

Recent progress in electron-beam resists for advanced mask-making by D.R.Medeiros, A.Aviram, C.R.Guarnieri, W.S.Huang, R.Kwong, C.K.Magg, A.P.Mahorowala, W.M.Moreau, K.E.Petrillo, and M.Angelopoulos

Important parameters Resolution (nm) Sensitivity (C/cm^2)

Resist limitations• Tendency of the resist to swell in the

developer solution.

• Electron scattering within the resist.– Broadens the diameter of the incident electron

beam.– Gives the resist unintended extra doses of

electron exposure .

Applications of Electron Beam Lithography

• Research- Nanopatterning on Nanoparticles- Nanowires- Nanopillars- Gratings- Micro Ring Resonators- Nanofluidic Channels

• Industrial / Commercial- Exposure Masks for Optical Lithography- Writing features

Nanopatterning on nanoparticles• Significance

- Photonic Crystals- Quantum Dots- Waveguides

• Electron Beam Lithography- Fine writing at moderate electron energies- 37nm thick lines with 90nm periodicity- 50nm diameter dots with 140nm periodicity

(2003), Patterning of porous Silicon by Electron Beam Lithography, S. Borini, A. M. Rossi, L. Boarino, G. Amato

Nanowires• Applications

- High-Density Electronics (Sensors, Gates in FETs)- Molecular Electronics & Medical/Biological Applications

• EBL with Electrochemical size reduction- High-Resolution Controlled Fabrication- Widths approaching 10nm regime

• Patterning of Films of Gold Nanoclusters with Electron Beam Direct Write Lithography- Sub 50nm wide Nanowires- Controlled thickness at single particle level

Controlled Fabrication of Silicon Nanowires by Electron beam lithography and Electro- chemical size reduction (2005), Robert Juhasz, Niklas Elfstrom and Jan Linnros

Nanometer Scale Petterinng of Langmuir-Blodgett Films of Gold Nanoparticles by Electron Beam Lithography (2001), Martinus H.V Werts, Mathieu Lambert, Jean-Philippe Bourgoin and Mathias Brush

Nanopillars• Significance

- Quantum Confinement Effects- Photoconductive response in Nanopillar arrays

• EBL and Reactive Ion Etching- Etched Pillars with 20nm diameter

Nanotechnology using Electron Beam Lithography, Center for Quantum Devices

Gratings• Applications

- Distributed Feedback Lasers- Vertical Cavity Surface Emitting Lasers

• Continuous Path Control Writing using EBL- Avoids stitching errors

Nanotechnology using Electron Beam Lithography, Center for Quantum Devices

Micro Ring Resonators• Applciations

- Optical Telecommunication and Networks

• EBL and Dry Etching- 105 devices/cm2 density

Nanotechnology using Electron Beam Lithography, Center for Quantum Devices

Nanofluidic Channels• Significance

- Laboratory on a chip

- Single Molecule Detection

• Electron Beam Lithography- Single step planar process

- Tubes with inner dimension of 80nm

(2005) A single-step process for making nanofluidic channels using electron beam lithography, J. L. Pearson and D. R. S. Cumming

Industrial Applications

• Exposure Masks for Optical Lithography using EBL

• Writing Features

Some Applications of E-Beam Lithography

• Cryo-electric devices• Optoelectronic devices• Quantum structures• Multi-gate Devices• Transport mechanism for semi and superconductor interfaces• Optical devices• Magnetism• Biological Applications

– Nano-MEMS– Nanofluidics

Future opportunities for electron beam lithography

1. Double gate FinFET devices2. Single electron transistors3. Photonic crystals

• Principle

Full control over a very

thin body region by two gates

• Fabrication thanks to e-beam

- Beam diameter smaller than 2nm

- Low energy (5 keV)

- High resolution organic resist

- Overlay accuracy thanks to scanning of registration marks

- Silicon etching

Double gate FinFET devices - Concept

20 nm electron beam lithography and reactive ion etching for the fabrication of double gate FinFET devices (2003), J. Kretz , L. Dreeskornfeld, J. Hartwich, W. Rosner

Nanoscale FinFETs for low power applications (2004), W. Rösner, E. Landgraf, J. Kretz, L. Dreeskornfeld, H. Schäfer, M. Städele,T. Schulz, F. Hofmann, R.J. Luyken, M. Specht, J. Hartwich, W. Pamler, L. Risch

• High performance devices

Transfer characteristic similar to

those obtained with bulk transistors

Appl: SRAM because high density

+ capability of driving a large bitline load

• Low power applications

High on-current, very low off-current

Double gate FinFET devices – Characteristics & Applications

Nanoscale FinFETs for low power applications (2004), W. Rösner, E. Landgraf, J. Kretz, L. Dreeskornfeld, H. Schäfer, M. Städele,T. Schulz, F. Hofmann, R.J. Luyken, M. Specht, J. Hartwich, W. Pamler, L. Risch

Single electron transistor - Concept• Physic principle

Weak external force to bring an additionalelectron to a small conductor “island”=> Repulsing electric field

• SET concept- Down-scaling- Low power consumption

• Difficulties- Need of very small “islands” becausethe addition energy must overload the temperature effects

- Polarization in case of impurities=> randomness background charge

Single-Electron Devices and Their Applications (1999), Konstantin K. Likharev

Single electron transistor - Fabrication

Fabrication of silicon nanowire structures based on proximity effects of electron-beam lithography (2003), S.F. Hua, W.C. Wengb, Y.M. Wanb

• Classic technique

Smallest “island” needed

=> Use of high resolution lithography technique

=> E-beam lithography

• With silicon nanowires

Lithography with e-beam, with specific beam current density and dose

Results: single electron charging effect

Polysilicon grain = “islands”

Grain boundaries = mini tunnel barriers

Single electron transistor - Applications

• Supersensitive electrometryVery small change of gate voltage=> measurable variation of IVery useful for physical experiments

• Single electron spectroscopy

• Replacing MosFET?

• Random access memory- Bit stored in large conductiveisland (floating gate)- Need of a sense amplifier=> association with FET amplifier- Very impressive density: 1011 bit/cm

NO !!!

Single-Electron Devices and Their Applications (1999), Konstantin K. Likharev

Photonic crystals - Concept

• Aim: propagation of light in a controllable manner

• => Optical “chips” with waveguides, cavities, mirrors, filters…Example of very compact quantumoptical integrated circuit:

• Need of a dielectric or metallic lattice, with adjustable parameters: geometry, dielectric constant…

Three-dimensional photonic crystals operating at optical wavelength region (2000), Susumu Noda

• Creation of the desired lattice

- With e-beam lithography at low beam energy (5keV)

- Negative resist. Ex: SU8-2000, with high refractive index (1,69) and good mechanical stability

• Results

A few mode are allowed to propagate, depending of the photonic crystal parameters

2D photonic crystals

Two-dimensional photonic crystal waveguide obtained by e-beam direct writing of SU8-2000 photoresist (2004), M. De Vittorio, M.T. Todaro, T. Stomeo, R. Cingolani, D. Cojoc, E. Di Fabrizio

3D photonic crystals• Several methods to create the lattice

- Wafer-fusion and alignment

techniqueEx: Layers of III-V semiconductors (AlGaAs…)

- XRay and e-beam lithography

• Introduction of defect states, light emitting elements…)

By wafer-fusion, two-resist process…

Three-dimensional photonic crystals operating at optical wavelength region (2000), Susumu NodaXRay and e-beam lithography of three dimensional array structures for photonics (2004), F. Romanato, E. Di Fabrizio,M. Galli