Reaneh- Methods of Deposition of Ferroelectrics

Question 1

Two fundamental categories of deposition techniques

Answer 1

Films formed by the in-situ formation and simultaneous crystallisation of the ceramic layer.
Films which are deposited as amorphous layers and which undergo post deposition crystallisation

Question 2

Examples of in-situ formation and simultaneous crystallisation methods

Answer 2

Metallorganic chemical vapour deposition (MOVCD).
In-situ reactive sputtering.
Pulsed laser deposition (PLD)

Question 3

Examples of post deposition crystallisation methods

Answer 3

Sol gel spin coating.

Cold sputtering

Question 4

Deposition methods with most commercial potential

Answer 4

Sol-gel spinning, MOVCD, sputtering (from both categories?)

Question 5

Sol gel spin coating for SBT

Answer 5

Need lots of reagents to get the initial Sr-Bi-Ta-sol. One drop of this is put in the centre of a plate which is spun around at 2000rpm to spread it into a thin layer. Then goes through drying and pyrolysis at 150 degrees and 400 degrees and the carbon compounds are removed using oxygen. Then heated to 650 degrees for crystallisation.

Question 6

How to control quality in sol gel spin coating

Answer 6

Need to transform the amorphous layer to the perovskite or Aurivillius phase whithout going through an intermediate step. Layer therefore needs to be heated quickly to the crystallisation temperature (500-700C)

Question 7

Growth of perovskite phase in PZT

Answer 7

PZT phase nucleates on surface of Pt. It grows upwards in columns. Unfortunately an intermediate pyrochlore/fluorite phase forms at the same time. Properties of PZT controlled by whether the pyrochlore is transformed completely to perovskite.

Question 8

Typical parts of MOCVD machine

Answer 8

Have various precursors that go into separate liquid flow control units and then all feed into a liquid injection/atomiser unit. This feeds into a vaporiser and then reactor unit which has a low pressure control system

Question 9

Precursors for MOCVD

Answer 9

They are the key to successful deposition of ferroelectric thin films in this method. Their chemistry is similar to that of sol-gel alkoxides and ideally double alkoxide compounds are used. These are subsequently hydrolysed in-situ. For more complex oxides the mixing of precursors is essential which brings issues like volatility and vapour pressure and concentrations of gas (partial pressures) inside the reaction chamber

Question 10

Hydrolysis of double alkoxides for MOCVD equation

Answer 10

ABnOk(OR)2(m-k)+(m-k)H2O=ABnOm+2(m-k)HOR

n, k, 2(m-k) subscript If come after something
The 2(m-k)HOR means that the alkoxide phase is eliminated

Question 11

Key steps to deposition from MOCVD

Answer 11

1. Evaporation of double/single alkoxide precursors and transportation to the vicinity of the substrate.
2. Controlled vapour phase hydrolysis of the precursors.
3. Polycondensation of the partially hydrolysed precursors onto substrate surface.
4. In situ crystallisation of the ferroelectric by controlled heating of the substrates in the deposition chamber.
   Gives very good quality

Question 12

Problem of MOCVD

Answer 12

For future integration the surface of the thin film need to be flat. MOCVD is a vapour phase technique so crystal facets after form on the surface of the sample.

Question 13

Features of a sputtering system

Answer 13

The sputtering target (starting material) is on a negative electrode in a vacuum chamber. There is a counter electrode somewhere else in the chamber. There is a vacuum pump and an inert gas inlet (Ar). Electrons and atoms come off the target material and Ar+ ions go towards the material. The target atoms removed hit the substrate on the counter electrode resulting in film growth.

Question 14

Details of sputtering steps

Answer 14

Usually carried out with a heated substrate (acts as the anode) and uses a reactive gas (O2) in the sputter plume. This ensures O content maintained at the stoichiometric value during deposition. Plasma is generated (usually Ar based) using a large DC or RF power supply. Beam of ions impacts the surface and removes atoms and molecules of the target. Substrate is negatively charged and attracts the molecules of the target which rearrange themselves in-situ on the substrate surface to form crystalline compound.

Question 15

What is the target material in sputtering?

Answer 15

Usually a compacted version of the desired oxide. Composition reflects not only the desired film type but often adjusted to take into account the sputtering rates

Question 16

What happens if substrate is kept cold in sputtering?

Answer 16

A post deposition crystallisation process is often required.

Question 17

Problems with sputtering

Answer 17

When the rate of sputtering of the metallic ions (easier ones to remove have more removed) in the target are different and the film composition differs from that intended. Fundamental problem is deposition of a uniform layer. Majority of target ions attracted to the centre of the substrate since this is the main direction of the plume of ions. Means film on edge of substrate thinner than at centre.