Uniform Thin Film Deposition
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Transcript of 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).