PHYSICAL VAPOR DEPOSITION

EVAPORATION PROCESS

ATOMS TO GAS STATE

VAPOR SOURCES

CONSTRUCTION AND USE OF VAPOR SOURCES

EVAPORATION RATE

TRANSPORT TO SURFACE

·          POINT SOURCE

·          SURFACE SOURCE

DEPOSIT ONTO SUBSTRATE

THICKNESS DISTRIBUTION

·          POINT SOURCE

·          SURFACE SOURCE

UNIFORMITY IN THICKNESS

FILM PURITY

Evaporation Process

Source material is transformed into gaseous state

Transport source atoms to substrate

Deposit atoms onto substrate

Binding on the surface of the substrate

Evaporation – Overview

Some sources evaporate from liquid

Some sources sublime from solid

Some sources break apart

Metal alloy sources do not give same alloy in films

Vapor sources

Should have very high melting point

Low vapor pressure at the deposition temperature

Should not react with the evaporant materials

Commonly used material sources are

Tungsten, Molybdenum, Tantalum

Shape: should be possible to hold the evaporant in the available form.

Construction and use of vapor sources

Hair pin (A) helix (B) Wire basket (C) sources

Evaporant materials available in the form of wire are used with these sources. They are held by its surface tension.

Evaporation takes place in all directions- examples of point source

Metal foil dimpled sources (D)- example of surface source

These sources are used for small evaporant quantities, made of tungsten, molybdenum or tantalum sheets. They are designed to have reduced width in the center to concentrate heat in the area of evaporant.

Evaporation rate (flux) from Kinetic theory

Transport to surface

The distribution of evaporant depends on the geometry of source

1. Point Source
2. Surface Source

Point Source

Thickness incase of a point source is

Surface Source

Thickness incase of a surface source is

Deposition onto substrate

Thickness distribution

Consider a flat substrate, perpendicular to source

And let d₀ and d are the thickness of deposits at a horizontal distance ‘l’, vertically from a source at a distance ‘h’

Point Source

Where d₀ is the maximum thickness which is

Surface Source

Uniformity in film thickness

Uniformity in thickness can be achieved by

i)                    by decreasing the sample size

ii)                  by increasing the distance to substrate

- For this we will need bigger chamber

-         We need better vacuum

iii)                put source and substrate on same sphere surface

iv)                use multiple sources

v)                  move substrate during deposition and

vi)                use rotating mask to reduce evaporant near center

Film Purity