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25 Cards in this Set
- Front
- Back
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top down vs bottom up |
large component and carve away material vs building something with smaller components |
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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 |
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advantages of bill milling
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-can be readily implemented commercially -used to make CNTs and boron nitride NTs -preferred method for NCs (eg CeO2, ZnO) |
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lithography |
defining geometric pattern into thin material (resist) which is radiation sensitive polymer |
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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) |
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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 |
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theoretical resolution of photolithography |
2bmin = 3(lamda(s+d/2))^1/2 |
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advantage of E-beam lithography
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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
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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 |
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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 |
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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 |
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FIB lithography |
by introducing gases or organic compound, FIB can selectively etch one material faster than surrounding materials or deposit metal/oxide
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purpose of etching |
-remove material in areas identified by lithography -create structures for functional use -remove oxide layers below features to allow for motion |
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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 |
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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 |
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types of dry etching |
chemical, physical or both |
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wet etching |
-redox equations define process
-most wet etching is isotropic -etch mixtures can change etch rate depending on silicon crystal orientation |
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advantages of wet etching
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-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 |
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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 |
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dry etch methods |
ion beam etch, gaseous chemical etch, plasma enhanced etch, reactive ion etch, deep reactive ion etch |
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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 |
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sputtering (ion milling or ion beam etch)
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-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 |
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ion milling |
-plasma etch has low selectivity -plasma etch anisotropic -high RF levels cause damage to wafer |
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reactive ion etch
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-combination of physical/chemical etching -both Ar and chemical gas used -Ar performs ion milling physical etch and chemical etch proceeds as well |
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advantages of reactive ion etch |
-anisotropic profile -higher etch rate -higher selectivity ratio than physical etch -smaller feature size possible -process of choice |
