lecture 2: assembly of nanostructures i€¦ · lithography - optical (used in almost all ic...

Nanoscience II spring 2009 1

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


Some examples

Top-down (planar)flip-up technique

Bottom-up: Au dots deposited bymicelles

Hybrid approach:


Self-organised SiGe dots on Si


Self-assembled lattice of calix[4]hydroquinone nanotubes with an 0.4 nm Ag wire inside


•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


Film deposition methods


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


Strain relaxation

Island formation

Thicker films: misfit dislocations


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


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


Lithography overview


Lithography principle

Methods:

• Contact

• Proximity

• Projection


Electron beam lithography- most widely used nanopatern writing technique- minimum dimension features written 5-20nm- serial and slow- mask making, prototyping, very limitedmanufacturing,- research


Electron-beam lithography:

Simulation of electron trajectories inPMMA resist on Si


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


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


Focused Ion Beam


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


Conventional,MultistepMicrofabricationvs.One Step FIBMicrofabrication


Producing a patterned dose ofions on the surface

Imaging

Focused ion beam:


Focused Ion Beam Instrumentation

FEI 620- Dual Beam, Electron & Ionwith Gas Feed


Focused Ion Beam Milled Cross Section ofPart of an Integrated Circuit


Nanoimprint lithographyHot embossing UV nanoimprint

Nanoscience II spring 2009 34From Motorola

Micro-contact printing

lecture 2: assembly of nanostructures i€¦ · lithography - optical (used in almost all ic...

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