THIN FILMS

DEFINITION

A thin film is a layer of material ranging from fractions of

a nanometer (monolayer) to several micrometers in

thickness.

• Physical vapor deposition

• Chemical vapor deposition

• Electrodeposition

• Langmuir-Blodgett

• Sol-gel method

METHODS OF APPLYING THIN FILMS

PHYSICAL VAPOUR DEPOSITION

The objective of these deposition processes is to controllably transfer atoms from

a source to a substrate where film formation and growth proceed atomistically.

IMPORTANT METHODS :

EVAPORATION

SPUTTERING

LASER DEPOSITION

EVAPORATION

In evaporation method , atoms are removed from the source by thermal means.

Until the late 1960s, evaporation clearly surpassed sputtering as the preferred film deposition technique.

Higher deposition rates, better vacuum, and, thus, cleaner environments for film formation and growth, and general applicability to all classes of materials were some of the reasons for the ascendancy of evaporation methods.

Two modes of evaporation can be distinguished in practice, depending on whether the vapor effectively emanates from a liquid or solid source. As a rule of thumb, a melt will be required if the element in question does not achieve a vapor pressure greater than lop3 torr at its melting point. Most metals fall intothis category, and effective film deposition is attained only when the source is heated into the liquid phase. On the other hand, elements such as Cr, Ti, Mo, Fe, and Si reach sufficiently high vapor pressures below the melting point and, therefore, sublime.

The vapour composition is usually different from that of the original solid or liquid source.

Reaction Type Chemical Reaction Examples Comments

Evaporation without

dissociation

MX(s or I) -+ MX( g) SiO, B,O,

GeO, SnO, A1N

CaFz, MgFz

Compound

stoichiometry

maintained in

deposit

Decomposition MX(s) ⟶t M(I) + (l/n)X,(g) III-V

semiconductors

Separate

sources are

required to

deposit these

compounds

Evaporation

with dissociation

a. Chalcogenides

X = S, Se, Te

MX(s) + M(g) + (1/2)Xz(g) CdS, CdSe

CdTe

Deposits are

metal-rich;

separate

sources are

required to

deposit these

compounds

b. Oxides

MO2 𝑠 ⟶ MO g + (1/2)O(g) SiO2,GeO2,TiO2,

SnO2,

ZrO2

Metal-rich

discolored

deposits;

dioxides are

best deposited

in 0, partial

pressure

(reactive

evaporation)

SPUTTERING

When the ion impact establishes a train of collision events in the

target leading to the ejection of a matrix atom.

DIFFERENT TYPES OF SPUTTERING:

DC Sputtering

RF Sputtering

Magnetron Sputtering

Reactive Sputtering

Bias Sputtering

1. Thermal evaporation mechanism

2. Low kinetic energy of evaporant

atoms (at 1200 K, E = 0.1 eV)

4. Directional evaporation according to cosine law

5. Fractionation of multicomponent alloys,

decomposition, and dissociation of compounds

6. Availability of high evaporation source purities

1. Ion bombardment and collisional

2. High kinetic energy of sputtered

4. Directional sputtering according to cosine law at

high sputter rates

5. Generally good maintenance of target stoichiometry,

but some dissociation of compounds.

6. Sputter targets of all materials are available; purity

varies with material

1. Evaporant atoms travel in high or ultrahightorr

2. 2. Thermal velocity of evaporant io5 cm/sec.

3. Mean-free path is larger than evaporant - substrate

spacing.

Evaporant atoms undergo no collisions in vacuum discharge

pressure discharge region

1. Sputtered atoms encounter high) ambient (- 100 mtorr)

2. 2. Neutral atom velocity - 5 x lo4

3. Mean-free path is less than targetsubstrate spacing.

Sputtered atoms undergo many collisions in the

1 . Condensing atoms have relatively

2. Low gas incorporation

3. Grain size generally larger than4. Few grain orientations (textured low energy

for sputtered film).

1. Condensing atoms have high energy

2. Some gas incorporation

3. Good adhesion to substrate4. Many grain orientations

CHEMICAL VAPOUR DEPOSITION

Chemical vapor deposition (CVD) is the process of chemically

reacting a volatile compound of a material to be deposited, with

other gases, to produce a nonvolatile solid that deposits

atomistically on a suitably placed substrate.

low-pressure (LPCVD),

plasma-enhanced (PECVD), and

laser-enhanced

(LECVD) chemical vapor deposition

SOL-GEL METHOD

”Formation of an oxide network through polycondensation reactions of a

molecular precursor in a liquid.”

A sol is a stable dispersion of colloidal particles or polymers in a solvent.

The particles may be amorphous or crystalline. An aerosol is particles in a gas

phase, while a sol is particles in a liquid,

A gel consists of a three dimensional continuous network, which encloses a

liquid phase, In a colloidal gel, the network is built from agglomeration of

colloidal particles. In a polymer gel the particles have a polymeric sub-structure

made by aggregates of sub-colloidal particles. Generally, the sol particles may

interact by van der Waals forces or hydrogen bonds. A gel may also be formed

from linking polymer chains. In most gel systems used for materials synthesis,

the interactions are of a covalent nature and the gel process is irreversible. The

gelation process may be reversible if other interactions are involved.

•The idea behind sol-gel synthesis is to “dissolve” the compound in a

liquid in order to bring it back as a solid in a controlled manner.

•Multi component compounds may be prepared with a controlled

stoichiometry by mixing sols of different compounds.

•The sol-gel method prevents the problems with co-precipitation, which

may be inhomogeneous, be a gelation reaction.

•Enables mixing at an atomic level.

•Results in small particles, which are easily sinterable

THANK YOU