MM669

Group-2

Ravi Kumar Prajapati

12D110035

Tensile stress evolution during deposition of Volmer-Weber thin film

Adatomadatom interactions are stronger than those of the adatom with the surface.

Islands are formed due to this interaction.

Islands grow and impinge to form a

network of islands.

Coalesce and form a polycrystalline film.

During coalescence stresses are

generated.

Volmer-Weber Deposition

Tensile stress generation

Nix-Clemens Model

Average stress

Zipping Distance

This model predicts values higher than those observed

in experiments

Finite Elemental Method(FEM) used to form a modal for

coalescence process.

Island was represented by a two dimensional element of

plain strain.

A series of displacement were imposed along the surface to a

height Z to mimic the zipping process.

Sum of strain energy and interfacial

energy represented as the change in

the total energy.

These energies were calculated as a

function of zipping distance and min.

value of the energy corresponds to

equilibrium zipping distance Z.

Values obtained from FEM modal were closer to values that are obtained by experiments.

Both the modals have different relation with radius of island.

At low substrate coverage an island will impinge on only

those island which has not impinged on any other island.

First coalescence was represented as either traction at the

island-substrate interface or sliding at island-substrate

interface.

In traction one island stretched towards the other island to

form a grain boundary and generate a tensile stress.

In sliding one island moves towards other island and

generate a small compressive stress

Traction Sliding

At higher substrate coverage islands coalesce with that

islands which has already coalesced.

In case of traction after second coalescence stress

approximately doubled.

In case of sliding after second coalescence stress is similar

to first coalescence by traction.

(i) And (ii) are first and second coalescence by traction

(iii) and (iv) are first and second coalescence by sliding

Stress Relaxation

Diffusion of the matter through the grain boundary and surface

Stress relaxation rate

Experimental stress measurement

Curvature was measured using multibeam optical

deflection technique.

By Stoneys equation Stress-thickness can be measured.

Results from Simulation

When two islands impinged, the equilibrium zipping distance and

stresses within the island were calculated using the FEM results

for the plane strain geometry with traction.

To calculate avg. stress, stress obtained from new coalescence is

added to existing avg. stress in the island.

Stress relaxation was assumed to occur via diffusion mechanism

Results from Simulation

In case of sliding two different types of coalescence occur.

First few coalescence results in slightly compressive

stress.

Further coalescence results in tensile stress.

Discussion

There are mainly three features of stress-thickness vs film

thickness curves

First-

Onset thickness of stress-thickness.

In simulation with traction the onset of stress-thickness

occurred at much smaller thickness than that was

observed in experiment.

With increment in temperature this onset onset thickness

increases.

In case of sliding this onset occurs at higher thickness

than both above cases.

Discussion

Second-

Maximum value of stress-thickness and corresponding

film thickness value.

Value of max. stress-thickness decreases and

corresponding film thickness increases as deposition

temperature increases.

Third-

Temperature dependence of the curve after maximum

value.

Slope of the curve is called rate of stress relaxation

This slope is decreases with increase in temperature.

summary

To compare with analytical modal a modal using FEM was

developed. It was found that it was more consistent than

analytical modal with experimental measurement.

Two types of island coalescence behaviour were

considered.

In situ measurement of curvature was performed and

stress-thickness calculated during experiment.

Simulation produces similar qualitative result to

experimental .

measured maximum stress-thickness decreased, and

occurred at a larger film thickness, with increasing

deposition temperature.

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