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Lecture 2: Assembly of nanostructures I
Top-down approachBased on conventional CMOS toechnology. Starts at the wafer level andpatterning proceeds by lithography and etching. Extension of the techniqueto the deep nanoscale possible by extreme UV or X-ray lithography, e-beamdirect write or focused ion beam. Also direct imprint. Extremely expensive.
Bottom-up approachFabrication of nanosized structures start from individual atoms andmolecules, which are ordered physically or reacted chemically to obtain thedesired features. Typically ordering is only over short ranges.
Hybrid approachesEmploys a combination of top-down approach for coarse definition of thepattern and a bottom-up technique to realize short-ranged ordered nano-scalestructures, which align to the coarser, in long-range order.
Nanotechnological approaches
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Some examples
Top-down (planar)flip-up technique
Bottom-up: Au dots deposited bymicelles
Hybrid approach:
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Self-organised SiGe dots on Si
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Self-assembled lattice of calix[4]hydroquinone nanotubes with an 0.4 nm Ag wire inside
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•Additive methods: include deposition of material on a substrate,assembly by e.g. gluing, soldering, bonding, as well as directed orself-controlled growth of desired features
• Subtractive methods: refer to the dedicated removal of material andinclude chemical and physical etching as well as mechanical millingand chipping techniques, or radiation-assisted ablation
• Modifying metods: in which material properties are changed, e.g.the concentration of carriers in semiconductors by doping, or themicro- or nanostructure of the of the phase (single-crystalline,polycrystalline, amorphous) by radiation.
Assembly methods in micro- andnanotechnology
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Film deposition methods
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Epitaxy
Homoepitaxy:
a) step-propagationb) 2D-island growthc) multi-layer growth
HeteroepitaxyGibb’s free energy:
Gs-vac > Gs-lay + Glay-vac :
Frank-van der Merwe growth
Gs-vac < Gs-lay + Glay-vac :
Volmer-Weber growth
Strained layer: island growthon wetting layer
Stranski-Krastanov
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Strain relaxation
Island formation
Thicker films: misfit dislocations
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Top-down assembly
Planar Fabrication:Three main processes:- pattern definition in resist (lithography)- transfer of pattern into surface or onto surface- material addition or material removal
Alternative: Resistless, direct pattern transfer by materialaddition or removal
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Lithography
- optical (used in almost all IC manufacturing)- electron beam (mask making, prototyping, very limitedmanufacturing, research)- next generation to replace optical:- extreme ultraviolet (EUV) ~13nm- electron beam projection- X-ray- ion beam projection- focussed ion beam- imprint
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Lithography overview
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Lithography principle
Methods:
• Contact
• Proximity
• Projection
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Electron beam lithography- most widely used nanopatern writing technique- minimum dimension features written 5-20nm- serial and slow- mask making, prototyping, very limitedmanufacturing,- research
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Electron-beam lithography:
Simulation of electron trajectories inPMMA resist on Si
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E-beam lithography example
The image to the left is a top view scanning electron microscope image of a fullyprocessed resonator structure. The structures are patterned by 3 kV electron beamlithography (EBL). The center image shows a top view scanning electron micrograph of areleased cantilever. Approximate dimensions of the cantilever are: 420 nm wide, 600 nmthick and 20 μm long. It is clearly seen that low energy EBL can generate high resolutionstructures. The image to the right is an SEM image from the front of an 83 nm thickchromium cantilever. Residues of silicon are seen underneath the cantilever.
Highly mass-sensitive resonator integrated with CMOS circuitry,Lund University
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Further examples, top-down, pre-patterning
V-groove quantum wire:
a) GaAs(001), V-grooves etchedin [01-1] directions, GaAsquantum wire buried inAlGaAs and vertical Qwires
b) close-up of crescent-shapedGaAs quantum wire
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Focused Ion Beam
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Fabrication: Electrons vs. Ions
electrons:- min. beam diam. ~1-5nm- current ~1-10pA- resist exposure(lithography)- beam induced chemistry,precursor gas- deposition- etching
ions:- min. beam diam. ~5-30nm- current ~ 1-10pA- milling- implantation- beam induced chemistry.precursor gas- deposition- etching
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Conventional,MultistepMicrofabricationvs.One Step FIBMicrofabrication
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Producing a patterned dose ofions on the surface
Imaging
Focused ion beam:
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Focused Ion Beam Instrumentation
FEI 620- Dual Beam, Electron & Ionwith Gas Feed
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Focused Ion Beam Milled Cross Section ofPart of an Integrated Circuit
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Nanoimprint lithographyHot embossing UV nanoimprint
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Micro-contact printing