Part 5 - Fabrication 2

Question 1

What is physical vapour deposition?

Answer 1

When a condensible vapour is produced by physical means and deposited on a solid substrate

Question 2

What is chemical vapour deposition?

Answer 2

If a volatile material or compound reacts with or without gases to produced a non-volatile solid film, this is chemical vapour deposition.

Question 3

What should the condition of the substrate be for vapour deposition?

Answer 3

The substrate should be placed at relatively low temp to meet saturation conditions.

Question 4

Define impingement rate

Answer 4

z = P / sqrt(2 pi m k T)

Question 5

Define supersaturation condition

Answer 5

S = Ji / z (T) - 1 > 0

Question 6

List and describe the methods to get a stoichiometric vapour

Answer 6

Flash evaporation. By dropping powders or grains of source material onto a hot surface. Vapour condenses rapidly onto a rel. cold surface with composition same as source material.

E-Gun - using an electron beam heated evaporator, source material evaporates from the molten end of the rod. In s.s, vapour comp = rod composition. Molten end of the rod must be enriched in the less volatile component. Automatic since diffusion of liquid is rapid.

Co-evaporation: Effective technique for the compositionally accurate deposition of compound semiconductor films whose components vapour pressure differ greatly. molecular beam epitaxy (MBE).

Sputtering - sputtering of certain material, whose ejected particles are molecules, was utilised to obtain a stoichiometric vapour. This involves ejecting material from a “target” that is a source onto a “substrate” such as a silicon wafer. The sputtering gas is often an inert gas such as argon.

Question 7

Define sputtering

Answer 7

Energetic ions from the plasma of a gaseous discharge bombard a target that is the cathode of the discharge. Target atoms are ejected and impinge on a substrate, forming a coating.

Question 8

What is the simplest evaporation source?

Answer 8

Thermal sources. this could still be when the energy supplied to the evaporate may be from protons or electrons, however the vapourising mechanism is thermal in nature.

Question 9

Give conditions to design an IDEAL effusion cell

Answer 9

Liquid and vapour phase in equilibrium with the cell
mean free path inside the cell is much greater than the orifice diameter
Orifice is flat
Wall thickness is much less than orifice diameter

See picture

Question 10

Describe near ideal effusion cell

Answer 10

Impossible to design an ideal cell. With a thick orifice lid, diffusion and specular reflection off the sidewalls are possible.

Non-equilibrium behaviour of vapour and liquid.

Question 11

Describe the E-Gun as an evaporation source

Answer 11

A target anode is bombarded with an electron beam given off by a charged tungsten filament under high vacuum. Electron beam causes atoms from the target to transform into the gaseous phase. These atoms then precipitate into solid form, coating everything in the vacuum chamber.

Question 12

Describe pulsed laser deposition

Answer 12

A high power pulsed laser beam is focussed inside a vacuum chamber to strike a target of the material that is to be deposited. This material is vaporised from the target (in a plasma plume) which deposits as a thin film on the substrate.

Question 13

Define the chemisorption coefficient

Answer 13

The fraction of adsorbed atoms transferred from physiosorption into chemisorption without re-evaporating.

Question 14

Define the condensation coefficient

Answer 14

The fraction of incident flux that actually condenses.

Question 15

Define the deposition rate by condensation

Answer 15

v = J / n …… J is the condensation flux (incident flux that actually condenses), n is particle density

Question 16

What are the 3 condensation growth modes?

Answer 16

Frank-van-der-Merwe-Wachstum - Atoms like the surface, form well into layers, epitaxy growth.

Volmer-Weber - Island growth, if the atoms like to interact with each other more than the surface. Not good for making a uniform film.

Stranski-Krastanow-Wachstum - Mixture of the above 2, where they like the surface and neighbouring molecules.

Question 17

What is non-epitaxy growth

Answer 17

Lots of islands

Question 18

What is epitaxial growth?

Answer 18

Layers

Question 19

Describe Molecular Beam Epitaxy (MBE)

Answer 19

Epitaxial growth. Interaction of on for several molecular or atomic beams that occurs on a surface of a heated crystalline substrate.

Question 20

Give advantages/disadvantages of MBE

Answer 20

Advantages:
Low temperature 
Precise control of doping 
Growth rate control is possible 
No boundary layer problems (growth rate is uniform) 

Disadvantages:
Expensive
Complex
Slow
Very high vacuum needed

Question 21

What is interesting about plasma?

Answer 21

It is an ionised gas. Electron temperature is very high (several thousands of degrees) but gas temperature can be low (room temp)

Question 22

What are attractive features of Self-assembly?

Answer 22

Proceeds spontaneously
Self-assembled structure is close to thermodynamic equilibrium.
Tends to have less defects, with self-healing capacity.

Question 23

Define CVD

Answer 23

Solid is grown by reaction of gaseous source materials and yielding a product effluent gas that is used to deposit conformal films.

Question 24

Give the procedure for CVD

Answer 24

Gases containing the required chemical elements are made to react in the vicinity of the substrate inside the reactor.

CVD usually performed at atmospheric or low pressure
Atmospheric pressure CVD gives poor product quality (thin film not continuous, lots of defects, maintaining stoichiometry difficult)

Can use high or low temperature depending on materials.

Question 25

Give applications for CVD

Answer 25

Coatings, semiconductors, optical fibres, catalysts.

Question 26

What is Plasma-enhanced CVD?

Answer 26

Plasma with reactive gases increases the film quality. The plasma is at a low temperature and pressure, but electrons are at high temperatures. This is technically “cheating”

Chemical reaction is involved for CVD, however this occurs after creation of a plasma of the reacting gases.

Question 27

How do you create plasma ?

Answer 27

Ionised gas. Two electrodes either side of the plate. Easier in vacuum, filling with nitrogen which ionise easier.

Question 28

Give advantages for PE-CVD

Answer 28

Low deposition temperature
High purity
High film density
Good film adhesion to substate, Easy control of reaction parameters

Question 29

Give disadvantages for PE-CVD

Answer 29

Expensive equipment, relatively, when compared to e.g. ball milling.
Small batch sizes.
Plasma bombardment is stressful (could burn)
Relatively slow.

Question 30

What is metal-organic chemical vapour deposition?

Answer 30

Produces single or polycrystalline films.
Uses moderate pressures.
Can build up many layers, each of a precisely controlled thickness, to create a material which has specific optical and electrical properties.

Question 31

What is the difference between MO-CVD and MBE

Answer 31

Growth of crystals is by chemical reaction rather than deposition

Question 32

Give the principles of operation for MO-CVD

Answer 32

Transport of precursor molecules by a carrier gas onto a substrate. Chemical reactions occur at the substrate surface.
Usual thermal decomposition temperature 300-500 degC

Question 33

Give advantages/disadvantages for MO-CVD

Answer 33

Advantages:
Flexible, can deposit semiconductors, metals etc
Very thin films < 1nm
Uniform epitaxial growth 
Fast growth rate 
Wide temperature control 

Disadvantages:
Highly toxic sources
Composition control not as precise as MBE
Large set of parameters (difficult to control)

Question 34

What is ALD?

Answer 34

Atomic Layer Deposition. Can make single atomic layer. Breaks the CVD reaction into two half-reactions, keeping the precursor materials separate during the reaction.
ALD film growth is self-limited and based on surface reactions making single atomic layers possible.

By keeping the precursors separate throughout the coating process, atomic payer thickness control of film grown can be obtained as fine as atomic level.

Question 35

What are the fundamental mechanisms of ALD?

Answer 35

Release of sequential precursor gas pulses to deposit a film one layer at a time.
A first precursor gas is introduced into the process chamber and produces a monolayer of gas on the wafter surface. Then a second precursor of gas is introduced into the chamber, reacting with the first precursor to produce a monolayer of film on the wafer surface.

• Chemisorption saturation process
• Sequential surface chemical reaction process.

*since each pair of gas pulses produces exactly one monolayer of film, the thickness of the resulting film may be precisely controlled by the number of deposition cycles.

Question 36

Give applications for ALD

Answer 36

Semiconductor memory
Cu interconnect barrier
Deposition in porous structures.

All applications take advantage of the uniformity, conformal step coverage, precise thickness control of deposited films.

Question 37

Give advantages/disadvantages for ALD

Answer 37

Advantages:
Stoichiometric films with large area uniformity and 3-D conformity
Precise thickness control
Low temperature deposition is possible
Gentle deposition process for sensitive substrates

Disadvantages:
Deposition rate slower than other CVD methods.
Range of materials that can be deposited by this method is less than MBE.

Question 38

Give the 2 general modes of nanoscale material synthesis

Answer 38

Dry methods: material is made in solid form from vapour phase precursors and used directly in the form it was made.

Wet methods: Materials are made by chemical reactions in solution or on a solid support, and separation of the desired material from unwanted solid or liquid materials is necessary.

Question 39

What are problems with nanoparticles?

Answer 39

They tend to form microscale agglomerates (clusters) - what is the point in nanoscale if they form clusters?

They can be dangerous to health if they are not immobilised.