Part 5 - Fabrication 1 Flashcards
What is a top down approach?
Starting with a larger component and carving away material. Patterning and etching away material as in building integrated circuits.
What is bottom up approach?
Building something by assembling smaller components. Self-assembly of atoms and molecules, as in chemical and biological systems.
List popular top-down methods
Mechanical attrition
Lithography
Etching
Give the essential information for ball milling
Used to produce metallic and ceramic nanomaterials.
Equipped with steel balls that rotate with high energy inside a drum and then fall on the solid with gravity force and crush material
Give advantages of ball milling
Readily implemented commercially
Used to make CNTs
preferred method for making metal oxide NCs
Cheap
What is lithography?
The process of defining a geometric pattern into a thin layer of material called resist which is a radiation sensitive polymer.
Give the process of lithography
(1) resist layer is spin coated or sprayed onto the wafer
(2) radiation is impinging on the resist
(3) Radiation changes the solubility of the revisit in a known solvent so called a developer.
What does resolution mean?
The smallest feature that can be obtained with high fidelity
Define Alignment accuracy
How accurately can we align successive masks on top of one another
Define throughput
The number of wafers that can be exposed / hr
Describe 2 common lithographic techniques
Photolithography and electron beam lithography.
Photolithography uses uv light to imprint an image on a wafer
EBL uses a beam of electrons that move over the wafer and writes the image over the photoresist.
Give the equation for minimum feature size
MFS = gamma/NA
NA = numerical aperture of device.
What method can you use to get small minimum feature sizes?
you need to reduce wavelength (gamma) of impinging radiation. It is popular to use deep UV light (~100 nm) or using electron beam lithography (EBL) with very very small wavelength.
Give the equation to calculate min theoretical resolution of EBL
b(min) = 3/2 sqrt[gamma (s + d/2)]
Gamma = wavelength of exposing radiation s = gap width between mask and photoresist surface d = photoresist surface
What are the challenges of EBL?
Electron beam lithography:
Charging effect - complicates the exact focussing of electron beam, with displacement possible
Proximity effect - scattering of electrons in resist film and substrate resulting in unwanted additional exposure. Can collide with other electrons and backscattered. Ruins the resolution.
Takes a long time.
What method can be used to overcome the problems of EBL?
Focussed ion beam lithography.
What are the advantages of Ion Beam Lithography
IBL ions have heavier mass than electrons Less proximity effect Less scattering effect High resolution Even smaller wavelength than E-beam.
What is the purpose of etching?
Removes material from areas identified by the lithography process
Creates structures for functional use
Remove oxide layers below features to allow for motion.
Define etch rate
the amount of material removed from the wafer over a defined time
Define uniformity
The evenness of the removal over the entire surface of water. Taking measurements across the wafter and the thickness should be the same. U = max-min/mean
Define Isotropic profile
Etching proceeds at equal rates in all directions
Define anisotropic profile
Etching proceeds fast in 1 plane
Define selectivity
Ability to distinguish between layers to be etched and material not to be etched.
Selectivity = Etchrate of sacrificial / Etchrate of structure
Low selectivity bad because everything is being etched.
Draw etch profiles
Isotropic and anisotropic (see notes)
Define wet etching
Immersion of entire wafer in liquid etchant solutions
Reaction between surface layer exposed and etchant
Chemical process
Often oxidation-reduction reactions.
Most wet etch processes are isotropic, they proceed in all directions equally.
Define dry etching
Placing wafer in chamber of chemical vapours or plasma
Can be both chemical and physical etch.
Give advantages of wet etching
Batch process Fast Used to remove sacrificial layers Good selectivity Cheap and easy to implement
Give disadvantages of wet etching
Problematic for feature sizes < 1 um
Stiction adherence of fabricated structure to the substrate during drying.
Contamination of wafer
Potentially dangerous
What is vapour etching?
Vapour etchants injected into process chamber and allowed to react. Generally isotropic. Much higher etch rate if using plasma enhancement
Give a method of dry etching
Ion beam etch, gaseous chemical reaction etc
What is plasma?
Ionised gas
What is sputtering (ion milling)?
Reduced pressure which increases the mean free path of molecules
Inert gas injected at low P and used as a milling tool. Bombarding the sample
What is plasma etching?
Plasma is generated in a chamber (ionised gas)
Energy transfer to molecules creates a plasma. +ve ions are bombarded to the -ve charged wafer target, removing molecules from the surface by brute force (physical removal)
What are the disadvantages of plasma etching?
Low selectivity
Tends to be anisotropic
High RF levels can damage the wafer
What is Reactive Ion Etch?
RIE combines both physical and chemical etching.
Both Ar and chemical gas used
Ar performs ion milling
Chemical performs chemical etch.
List the advantages of RIE
Less anisotropic profile
Higher etch rate
Higher etch selectivity
Smaller feature sizes are possible
Define a bottom up approach
Adding atom-to-atom, molecule-to-molecule
Self-Assembly
What are the challenges of the bottom-up fabrication?
Growing the structures where you want them
Making complicated patterns
Fabricating robust structures
Define self-assembly
A reversible process in which pre-existing parts or disordered components of a pre-existing system form structures of patterns.
Self-assembly can occur spontaneously in nature.
What are the two approaches of self-assembly?
Intermolecular and Intramolecular
Define crystallisation
Phase-change process in which solid crystals precipitate from a solution, melt or even from gases.
What is the underlying mechanism for crystallisation?
To provoke crystallisation, the state of a solution is shifted from under-saturated to supersaturated by an external action.
Give the thermodynamics (grad chemical potential) for supersaturation
see notes
What is the supersaturation ratio
S = a/a_eq or x/x_eq or c/c_eq
Define relative supersaturation
sigma = delta_c / c_eq
Define absolute supersaturation
delta_c = c - c_eq
Define undercooling
delta T = T_m - T
What is the difference between homogeneous and heterogeneous nucleation?
Homogenous - you can make it in the middle of the vessel, you don’t need any extra help for nucleation to start.
Heterogenous - if you have a surface it lowers the energy required for crystallisation to start. Need extremely pure system to use this because heterogenous nucleation also occurs if you have impurities.
What is very important to initiate nucleation?
Need to overcome an energy barrier (deltaG) to start nucleation.
What is the critical crystal nucleate size?
Where the gibbs free energy is a maximum.
What can be used to aid nucleation?
Nucleation seeds.
Define Chemical Vapour Deposition
If volatile compound of a material react with or without other gases to produce a non-volatile solid film, this is chemical vapour deposition
Define Physical Vapour Deposition
If condensible vapour is produced by physical means
Define the impingement rate
Number of collisions per unit area per second that a gas makes with a surface. z = P / sqrt(2 pi m k T).
Give the supersaturation condition
S = J/z_eq (T_sub) - 1 > 0
