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top down vs bottom up

large component and carve away material vs building something with smaller components

ball milling

-mills rotate, partially filled with material to be ground + grinding medium (WC or steel balls)


-balls rotate with high energy and fall on solid crushing it into nano crystallites


-mainly produces metallic/ceramic NMs

advantages of bill milling

-can be readily implemented commercially


-used to make CNTs and boron nitride NTs


-preferred method for NCs (eg CeO2, ZnO)

lithography

defining geometric pattern into thin material (resist) which is radiation sensitive polymer

3 step process of lithography

1. resist spin-coated or sprayed onto wafer

2. radiation (photons/electrons) impinged on resist


3. radiation changes solubility of resist in known solvent (developer)



3 important characteristics of lithography

-resolution: smallest feature with high fidelity

-alignment accuracy: how accurately can we align successive masks on top of one another


-throughput: number of wafers exposed per hour

theoretical resolution of photolithography

2bmin = 3(lamda(s+d/2))^1/2

advantage of E-beam lithography
wavelength of e-beam smaller than UV light so can be focused to a few nm in diameter and deflected accurately and precisely over a surface

7 steps in e-beam lithography (example)

1. e-beam lithography and developer


2. metallisation


3. lift off


4. resist application


5. photolithography and developer


6. metallisation


7. lift off

challenges with e-beam lithography

-charging effect: complicate exact focusing of beam and leads to displacement/distortion of exposed structures


-proximity effect: scattering of electrons in resist and substrate leads to unwanted exposure

advantages of ion beam lithography

-ions have heavier mass than electrons


-less proximity effect than e-beam


-less scattering effect


-higher res. patterning than UV/e-beam


-smaller wavelength than e-beam

FIB lithography

by introducing gases or organic compound, FIB can selectively etch one material faster than surrounding materials or deposit metal/oxide

purpose of etching

-remove material in areas identified by lithography


-create structures for functional use


-remove oxide layers below features to allow for motion

4 important characteristics of lithography

-etch rate: amount of material removed over period of time


-uniformity: evenness of removal over surface


-profile: isotropic or anisotropic


-selectivity: ability of etch to distinguish between layers to be etched and not

wet vs dry etching

-wet: immersing wafers in liquid etchant (reaction between exposed surface layer and etchant which is purely chemical)


-dry: placing wafer in chamber and pumping chemical vapours or plasma

types of dry etching

chemical, physical or both

wet etching

-redox equations define process

-most wet etching is isotropic


-etch mixtures can change etch rate depending on silicon crystal orientation

advantages of wet etching

-can be batch


-can be used to remove sacrificial layers in MEMS devices


-photo resist can be used as mask layer


-cheap/easy to implement


-good selectivity

disadvantages of wet etching

-problematic for < 1 micrometer


-striction (adherence of structure to substate during drying)


-potential contamination of wafer


-etch oxide with HF - dangerous

dry etch methods

ion beam etch, gaseous chemical etch, plasma enhanced etch, reactive ion etch, deep reactive ion etch

vapour etching

-gaseous forms of etchants injected into process chamber


-reactivity between Fl with surface occurs


-isotropic


-much higher etch rate can result with plasma enhancement

sputtering (ion milling or ion beam etch)

-reduced pressure, increases mean free path, fewer collisions


-inert gas injected at low P used as milling tool


-RF plasma in chamber: energy transfer to gas creates plasma of equal numbers of ions and molecules. Positive ions bombard negatively charged wafer removing molecules from surface

ion milling

-plasma etch has low selectivity


-plasma etch anisotropic


-high RF levels cause damage to wafer

reactive ion etch

-combination of physical/chemical etching


-both Ar and chemical gas used


-Ar performs ion milling physical etch and chemical etch proceeds as well

advantages of reactive ion etch

-anisotropic profile


-higher etch rate


-higher selectivity ratio than physical etch


-smaller feature size possible


-process of choice