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SB.Nano > Part 5 - Fabrication 1 > Flashcards

Part 5 - Fabrication 1 Flashcards

1
Q

What is a top down approach?

A

Starting with a larger component and carving away material. Patterning and etching away material as in building integrated circuits.

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2
Q

What is bottom up approach?

A

Building something by assembling smaller components. Self-assembly of atoms and molecules, as in chemical and biological systems.

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3
Q

List popular top-down methods

A

Mechanical attrition
Lithography
Etching

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4
Q

Give the essential information for ball milling

A

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

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5
Q

Give advantages of ball milling

A

Readily implemented commercially
Used to make CNTs
preferred method for making metal oxide NCs
Cheap

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6
Q

What is lithography?

A

The process of defining a geometric pattern into a thin layer of material called resist which is a radiation sensitive polymer.

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7
Q

Give the process of lithography

A

(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.

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8
Q

What does resolution mean?

A

The smallest feature that can be obtained with high fidelity

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9
Q

Define Alignment accuracy

A

How accurately can we align successive masks on top of one another

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10
Q

Define throughput

A

The number of wafers that can be exposed / hr

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11
Q

Describe 2 common lithographic techniques

A

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.

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12
Q

Give the equation for minimum feature size

A

MFS = gamma/NA

NA = numerical aperture of device.

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13
Q

What method can you use to get small minimum feature sizes?

A

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.

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14
Q

Give the equation to calculate min theoretical resolution of EBL

A

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
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15
Q

What are the challenges of EBL?

A

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.

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16
Q

What method can be used to overcome the problems of EBL?

A

Focussed ion beam lithography.

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17
Q

What are the advantages of Ion Beam Lithography

A 
IBL ions have heavier mass than electrons 
Less proximity effect
Less scattering effect
High resolution 
Even smaller wavelength than E-beam.
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18
Q

What is the purpose of etching?

A

Removes material from areas identified by the lithography process
Creates structures for functional use
Remove oxide layers below features to allow for motion.

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19
Q

Define etch rate

A

the amount of material removed from the wafer over a defined time

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20
Q

Define uniformity

A

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

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21
Q

Define Isotropic profile

A

Etching proceeds at equal rates in all directions

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22
Q

Define anisotropic profile

A

Etching proceeds fast in 1 plane

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23
Q

Define selectivity

A

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.

24
Q

Draw etch profiles

A

Isotropic and anisotropic (see notes)

25
Q

Define wet etching

A

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.

26
Q

Define dry etching

A

Placing wafer in chamber of chemical vapours or plasma

Can be both chemical and physical etch.

27
Q

Give advantages of wet etching

A 
Batch process
Fast
Used to remove sacrificial layers 
Good selectivity 
Cheap and easy to implement
28
Q

Give disadvantages of wet etching

A

Problematic for feature sizes < 1 um
Stiction adherence of fabricated structure to the substrate during drying.
Contamination of wafer
Potentially dangerous

29
Q

What is vapour etching?

A

Vapour etchants injected into process chamber and allowed to react. Generally isotropic. Much higher etch rate if using plasma enhancement

30
Q

Give a method of dry etching

A

Ion beam etch, gaseous chemical reaction etc

31
Q

What is plasma?

A

Ionised gas

32
Q

What is sputtering (ion milling)?

A

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

33
Q

What is plasma etching?

A

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)

34
Q

What are the disadvantages of plasma etching?

A

Low selectivity
Tends to be anisotropic
High RF levels can damage the wafer

35
Q

What is Reactive Ion Etch?

A

RIE combines both physical and chemical etching.
Both Ar and chemical gas used
Ar performs ion milling
Chemical performs chemical etch.

36
Q

List the advantages of RIE

A

Less anisotropic profile
Higher etch rate
Higher etch selectivity
Smaller feature sizes are possible

37
Q

Define a bottom up approach

A

Adding atom-to-atom, molecule-to-molecule

Self-Assembly

38
Q

What are the challenges of the bottom-up fabrication?

A

Growing the structures where you want them
Making complicated patterns
Fabricating robust structures

39
Q

Define self-assembly

A

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.

40
Q

What are the two approaches of self-assembly?

A

Intermolecular and Intramolecular

41
Q

Define crystallisation

A

Phase-change process in which solid crystals precipitate from a solution, melt or even from gases.

42
Q

What is the underlying mechanism for crystallisation?

A

To provoke crystallisation, the state of a solution is shifted from under-saturated to supersaturated by an external action.

43
Q

Give the thermodynamics (grad chemical potential) for supersaturation

A

see notes

44
Q

What is the supersaturation ratio

A

S = a/a_eq or x/x_eq or c/c_eq

45
Q

Define relative supersaturation

A

sigma = delta_c / c_eq

46
Q

Define absolute supersaturation

A

delta_c = c - c_eq

47
Q

Define undercooling

A

delta T = T_m - T

48
Q

What is the difference between homogeneous and heterogeneous nucleation?

A

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.

49
Q

What is very important to initiate nucleation?

A

Need to overcome an energy barrier (deltaG) to start nucleation.

50
Q

What is the critical crystal nucleate size?

A

Where the gibbs free energy is a maximum.

51
Q

What can be used to aid nucleation?

A

Nucleation seeds.

52
Q

Define Chemical Vapour Deposition

A

If volatile compound of a material react with or without other gases to produce a non-volatile solid film, this is chemical vapour deposition

53
Q

Define Physical Vapour Deposition

A

If condensible vapour is produced by physical means

54
Q

Define the impingement rate

A

Number of collisions per unit area per second that a gas makes with a surface. z = P / sqrt(2 pi m k T).

55
Q

Give the supersaturation condition

A

S = J/z_eq (T_sub) - 1 > 0

SB.Nano (8 decks)

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