Main group compounds in the electronics industry

Question 1

Thin film

Answer 1

A layer of material ranging from fractions of a nanometre (monolayer) to several micrometres in thickness
Typically a few 1000 A thick

Question 2

Uses of thin films

Answer 2

Electronic devices (doped Si, Ge etc)
LED lasers
Photovoltaics
Windows (SnO2:F)

Question 3

Thin film deposition techniques

Answer 3

Physical:
1. Evaporation
2. Sputtering
Chemical:
1. Chemical vapour deposition (CVD) / Metal-organic chemical vapour deposition (MOCVD)
2. Atomic layer deposition (ALD)

Question 4

Chemical vapour deposition

Answer 4

= the deposition of a thin film onto a surface via the chemical reactions of gaseous molecules that contain the atoms needed for the film material
= the deposition of thin films by pyrolysis of a suitable chemical precursor/combination of precursors in which all the unwanted parts of the molecule are burned away

Question 5

Chemical vapour deposition process

Answer 5

1. Process begins with one or more tanks containing the monomers/building blocks of the desired polymer coating. Can also have a tank containing an initiator
2. Materials are vapourised either by heating or by reducing the pressure, and are then introduced into a vacuum chamber that contains the material to be coated
3. The hot gaseous material naturally forms a coating on the colder substrate
4. Any volatile by-products produced are removed by gas flow through the reaction chamber
   (i. e. there is a carrier gas that makes up the main part of the gas volume in the process, analogous to solvent in liquid phase reactions)

Question 6

Purpose of initiator in CVD

Answer 6

Helps to speed up the rate at which the monomers links together to form polymer chains on the surface of the substrate

Question 7

Dual source method for deposition InAs (III-V material)

Answer 7

In(C2H5)3 + AsH3 —(~500 C)—> InAs + 3C2H6

Question 8

Examples of CVD/MOCVD by dual/multisource precursors

Answer 8

See flashcard for equations

Question 9

MOCVD

Answer 9

A specific type of CVD that uses metal-organic (i.e. organometallic) precursors
Can be extended to include precursors that contain metal-oxygen bonds, metal-nitrogen bonds or metal hydrides

Question 10

Chemical reactions in CVD

Answer 10

Can take place in the gas phase or on the surface

Question 11

Reasons why adsorbates can react rapidly on the surface

Answer 11

1. Bonds in the adsorbate are weakened
2. Presence of co-reagents on the surface
3. Adsorbate molecules/fragments may be mobile on the surface and may sample a variety of surface sites via surface diffusion, which may lead to reactions

Question 12

Surface diffusion

Answer 12

The process that ultimately leads to formation of a film of the target material

Question 13

Why are organometallics good precursors for the metal components of a thin film?

Answer 13

Because of their high volatility compared to purely inorganic species such as halides/oxides

Question 14

Requirements for CVD precursors

Answer 14

1. High volatility/high vapour pressure
2. High purity, easy to handle/store (adducts are good for this)
3. Clean decomposition e.g. alpha and beta-H elimination reactions
4. Cheap (depends on availability/abundance of metal)

SEE FLASHCARDS

Question 15

High volatility/vapour pressure property of CVD precursors

Answer 15

Compound ideally needs to be a gas (e.g. SiH4) but liquids and low-melting solids are ok
Need to minimise intermolecular interactions by using bulky ligands (but not too large - would add too much MW)
Chelating groups to block multiple coordination sites
Fluorinated ligands (adds a ‘non-stick’ property)

Question 16

Single-source precursors

Answer 16

= single molecules that contain all the components of the desired film
Therefore can be used alone in a CVD process
See flashcards for examples

Question 17

AACVD

Answer 17

Aerosol-assisted CVD

Diagram on flashcards

Question 18

Advantages of AACVD

Answer 18

CVD/MOCVD require the chemical precursor(s) to be sufficiently volatile at atmospheric pressure (APCVD) or under low pressure (LPCVD)
AACVD is an alternative technique that allows CVD with involatile precursors - it just depends on the solubility of the precursor
Eliminates the need for high vacuum/temperature, so is good for thermally unstable precursors

Question 19

AACVD process

Answer 19

An ultrasonic humidifier is used to generate micron- and submicron-sized aerosol droplets of the precursor solution
The aerosol is then moved into the reactor by a flow of N2 gas
At the high temperature in the reactor (approx. 300-400 C), the solvent aerosol evaporates, allowing the precursors, which are still in the gas phase, to adsorb onto the substrate

Question 20

Laminar flow

Answer 20

When a fluid flows in parallel layers, with no disruption between the layers

Question 21

ALD

Answer 21

Atomic layer deposition
A subclass of CVD
A method of applying thin films to various substrates with atomic scale precision
More refined than a typical CVD process

Question 22

Difference between ALD and CVD

Answer 22

ALD is similar in chemistry to CVD, except ALD breaks the CVD reaction into two half reactions
The precursor materials are kept separate during the reaction and released sequentially to deposit the film one layer at a time - i.e. the substrate surface is exposed to the precursors alternately and the precursors are never present simultaneously in the reactor

Question 23

ALD film growth

Answer 23

Film growth is self-limiting (i.e. the reaction terminates when all reactive sites on the substrate have been used up) and based on surface reactions
As a result, the maximum amount of material deposited on the surface after a single exposure to all the precursors is determined by the nature of the precursor-surface reaction

Question 24

ALD cycle

Answer 24

After the surface has been exposed to all precursors

Question 25

ALD process

Answer 25

1. Substrate exposed to a pulse of precursor 1
2. After precursor 1 has adsorbed onto the substrate surface, any excess precursor 1 is removed from the reaction chamber by purging
3. Precursor 2 is added to the reaction chamber and reacts with precursor 1 to create another layer on the substrate
4. Any excess precursor 2 is then removed from the chamber and the cycle is repeated until the desired thickness is achieved

Question 26

Al2O3 deposition equations

Answer 26

Al(CH3)3(g) + >Al-O-H(s) —> >Al-O-Al(CH3)2(s) + CH4(g)

2H2O(g) + >Al-O-Al(CH3)2(s) —> >Al-O-Al(OH)2 + 2CH4(g)

Question 27

Major advantages of ALD

Answer 27

Thickness of the material deposited is determined simply by the number of deposition cycles
Precursors are saturatively chemisorbed - produces stoichiometric films with large area uniformity and 3D conformality
Deposition can occur at low temperature, so provides a gentle deposition process for sensitive substrates

Question 28

ALD window graph

Answer 28

Draw

Question 29

Precursor requirements for ALD

Answer 29

(More stringent than for CVD)
Precursors must be volatile and thermally stable
Preferably liquids and gases
Should be sufficiently reactive that they can chemisorb onto the surface, but not so reactive that they react with themselves or start to etch/dissolve into the film/substrate
Need a short saturation time, good deposition rate and no gas phase reactions

Question 30

Comparison of CVD and ALD precursors

Answer 30

Flashcard