Tools Of Micro & Nano Fabrication

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 Silicon Wafer

expose to UV light

Exposed region becomes more soluble the pattern is reproduced on the resist Underlying semiconductor material etched by acid Solvents remove resist

Optical Lithography • Photoresistive resine • Patterns: Masks • Wavelenght resolution dependant

Resolution Limits • Contact Advantages: Good resolution

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Drawbacks: Masks thin and flexible ● Use ->defects ●

Resolution Limits • Proximity Advantages: Masks lifetime high

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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 ●

RESOLUTION • 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

Higher resolution lithography • X-Ray – 1-1.5nm range

lack of

refractive x-ray optics

• Extreme UV Lithography 14nm wavelength

10-

Extreme Ultraviolet Lithography • Small wavelength Better resolution • No lenses: mirrors • Laser plasma sources • 10 nm

Nanoimprint • techniques: Heat resine Cool down UV radiations

Patterning Techniques EUV soon in fabrication Nanoimprint E beam for 22nm X Rays difficult

The electron beam lithography

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Types of EBL  Electron Beam Direct Write  Electron Projection Lithography

Bragg-Fresnel lens for x-rays Paul Scherrer Institute

Electron Beam Direct Write • • •

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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 Write Types 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 Electron Beam Lithography Direct Write Electron Projection Lithography

New solutions

Limited throughput

Huge penetration depth of electrons

• SCALPEL (Bell Laboratories and Lucent technologies) 1995 • PREVAIL (IBM) 1999

Electron Projection Lithography • SCALPEL – High contrast – Image reduction

• PREVAIL – Larger effective field

Electron beam resists

1. Important parameters 2. Types of resist 3. Resist limitations

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

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

Resist limitations •

Tendency of the resist to swell in the developer solution.

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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

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Nanopatterning on Significance nanoparticles - Photonic Crystals - Quantum Dots - Waveguides

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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

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

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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

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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

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Continuous Path Control Writing using EBL - Avoids stitching errors

Nanotechnology using Electron Beam Lithography, Center for Quantum Devices

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Micro Ring Resonators

Applciations

- Optical Telecommunication and Networks

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EBL and Dry Etching - 105 devices/cm2 density

Nanotechnology using Electron Beam Lithography, Center for Quantum Devices

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Nanofluidic Channels

Significance

- Laboratory on a chip - Single Molecule Detection

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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

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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 devices 2. Single electron transistors 3. Photonic crystals

Double gate FinFET devices - Concept •

Principle Full control over a very thin body region by two gates

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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

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

Double gate FinFET devices – Characteristics & Applications • 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

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 additional electron to a small conductor “island” => Repulsing electric field

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SET concept - Down-scaling - Low power consumption

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Difficulties - Need of very small “islands” because the 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

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Single electron transistor Fabrication Classic technique Smallest “island” needed => Use of high resolution lithography technique => E-beam lithography

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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

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

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Single electron transistor Applications Supersensitive electrometry Very small change of gate voltage => measurable variation of I Very useful for physical experiments

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Single electron spectroscopy

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Replacing MosFET?

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Random access memory - Bit stored in large conductive island (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 quantum optical 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

2D photonic crystals • 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

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 technique Ex: 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 Noda XRay and e-beam lithography of three dimensional array structures for photonics (2004), F. Romanato, E. Di Fabrizio,M. Galli