Part 5 - Fabrication 1

Question 1

What is a top down approach?

Answer 1

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

Question 2

What is bottom up approach?

Answer 2

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

Question 3

List popular top-down methods

Answer 3

Mechanical attrition
Lithography
Etching

Question 4

Give the essential information for ball milling

Answer 4

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

Question 5

Give advantages of ball milling

Answer 5

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

Question 6

What is lithography?

Answer 6

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

Question 7

Give the process of lithography

Answer 7

(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.

Question 8

What does resolution mean?

Answer 8

The smallest feature that can be obtained with high fidelity

Question 9

Define Alignment accuracy

Answer 9

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

Question 10

Define throughput

Answer 10

The number of wafers that can be exposed / hr

Question 11

Describe 2 common lithographic techniques

Answer 11

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.

Question 12

Give the equation for minimum feature size

Answer 12

MFS = gamma/NA

NA = numerical aperture of device.

Question 13

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

Answer 13

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.

Question 14

Give the equation to calculate min theoretical resolution of EBL

Answer 14

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

Question 15

What are the challenges of EBL?

Answer 15

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.

Question 16

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

Answer 16

Focussed ion beam lithography.

Question 17

What are the advantages of Ion Beam Lithography

Answer 17

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

Question 18

What is the purpose of etching?

Answer 18

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

Question 19

Define etch rate

Answer 19

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

Question 20

Define uniformity

Answer 20

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

Question 21

Define Isotropic profile

Answer 21

Etching proceeds at equal rates in all directions

Question 22

Define anisotropic profile

Answer 22

Etching proceeds fast in 1 plane

Question 23

Define selectivity

Answer 23

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.

Question 24

Draw etch profiles

Answer 24

Isotropic and anisotropic (see notes)

Question 25

Define wet etching

Answer 25

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.

Question 26

Define dry etching

Answer 26

Placing wafer in chamber of chemical vapours or plasma | Can be both chemical and physical etch.

Question 27

Give advantages of wet etching

Answer 27

``` Batch process Fast Used to remove sacrificial layers Good selectivity Cheap and easy to implement ```

Question 28

Give disadvantages of wet etching

Answer 28

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

Question 29

What is vapour etching?

Answer 29

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

Question 30

Give a method of dry etching

Answer 30

Ion beam etch, gaseous chemical reaction etc

Question 31

What is plasma?

Answer 31

Ionised gas

Question 32

What is sputtering (ion milling)?

Answer 32

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

Question 33

What is plasma etching?

Answer 33

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)

Question 34

What are the disadvantages of plasma etching?

Answer 34

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

Question 35

What is Reactive Ion Etch?

Answer 35

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

Question 36

List the advantages of RIE

Answer 36

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

Question 37

Define a bottom up approach

Answer 37

Adding atom-to-atom, molecule-to-molecule | Self-Assembly

Question 38

What are the challenges of the bottom-up fabrication?

Answer 38

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

Question 39

Define self-assembly

Answer 39

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.

Question 40

What are the two approaches of self-assembly?

Answer 40

Intermolecular and Intramolecular

Question 41

Define crystallisation

Answer 41

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

Question 42

What is the underlying mechanism for crystallisation?

Answer 42

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

Question 43

Give the thermodynamics (grad chemical potential) for supersaturation

Answer 43

see notes

Question 44

What is the supersaturation ratio

Answer 44

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

Question 45

Define relative supersaturation

Answer 45

sigma = delta_c / c_eq

Question 46

Define absolute supersaturation

Answer 46

delta_c = c - c_eq

Question 47

Define undercooling

Answer 47

delta T = T_m - T

Question 48

What is the difference between homogeneous and heterogeneous nucleation?

Answer 48

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.

Question 49

What is very important to initiate nucleation?

Answer 49

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

Question 50

What is the critical crystal nucleate size?

Answer 50

Where the gibbs free energy is a maximum.

Question 51

What can be used to aid nucleation?

Answer 51

Nucleation seeds.

Question 52

Define Chemical Vapour Deposition

Answer 52

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

Question 53

Define Physical Vapour Deposition

Answer 53

If condensible vapour is produced by physical means

Question 54

Define the impingement rate

Answer 54

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

Question 55

Give the supersaturation condition

Answer 55

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