Uniform Thin Film Deposition

7/29/2019 Uniform Thin Film Deposition

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Thermal Vapor Deposition

How to obtain a thin film of uniform thickness

over a substrate of considerable size


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Thermal Vapor Deposition

A basic technique for thin film deposition over a

substrate.

Source material is heated in high vaccum.

The evaporated atoms get impinged over the substratesurface.

The evaporation rate is given by:


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Film Thickness and Uniformity

Uniform film thickness is essential in case of

microelectronics - integrated circuits, optical

coatings.

Accuracy of about 1% is required.

Understanding deposition geometry is important.


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Deposition Geometry

Characteristics of the source and orientation &

placement of the substrate.

Consider evaporation from a point source.

Total evaporated mass:

Mass deposition (substrate):

Deposition depends on:

geometric orientation ().

source-substrate distance.

dAc = dAscos


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Deposition Geometry

Similarly for a surface source, we have the following

expression:

Deposition rate depends upon:

Source-substrate distance

Emission angle and orientation ( and )

and are equal if both surfaces are parallel


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Uniform Thickness

Difficulties arise while dealing with substrates having a

large surface area or curved surface

Consider evaporation from a point source to a planar

substrate.

The film thickness d =

Thickest deposit (d0) occurs at l = 0.


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Similarly, for the surface source:


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We can utilize the film thickness distribution to design

source-substrate geometries for coating.

Example 1 : Coating of 150cm wide substrate using two

point sources. Tolerance: 10%

Parameters: Distance of sources from the centre line (D) and the

distance between source and substrate (hv)

A superposition of the solution for individual sources can provide

the graphical variation of thickness as a function of D and h Range for d/d0 is between 0.9 and 1.1

D/hv = 0.6 is optimum, maximum allowed value of r/hv = 0.87

Since r = 150/2 = 75cm, hv = 75/0.87 = 86.2 cm

Further, distance between the sources (2D) = 2x0.6x86.2 =

103.4 cm

Other solutions obviously exist, but we are seeking minimum

value of hv , to minimise waste of evaporant.


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Example 2 : Composite optical coating where a 1%

film thickness variation is required in each layer.

Diameter of substrate = 2r = 25 cm.

Substrate is rotated to even out source distribution anomalies.

Necessary fixturing requires that the sources be offset from the

axis of rotation by a distance R = 20 cm

How high (hv) should the rotating substrate be placed from the

source for optimum deposition? Film thckness variation is a complicated function of 3-D

geometry.

The graph suggests hv/R = 1.33 in conjunction with r/R = 0.6 is

suitable. (deviation between -0.6% to +0.5%)

required distance is hv = 1.33 x 20 = 26.6 cm


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Another way to acheive thickness uniformity is to haveboth the surface source and substrate on the surface ofa sphere as shown:

We Have:

Mass deposited is constant.

There are limitations.

Other methods:

Varying the location of source

Use of shutters

Controlled by a computer


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References:

Milton Ohring, Materials Science of Thin Films, 2ndedition, Academic Press, (1992).

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Uniform Thin Film Deposition