20141103 Thin Film Physics 2

8/10/2019 20141103 Thin Film Physics 2

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1FNNLFunctional Nanostructures & Nanoelectronics Lab

Semiconductor processing

Epitaxy


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Epitaxial growth

Adatom movement is critical for epilayer growth At high temperature, low flux, epilayer is formed

Surface should be very clean: eg) with native oxide, poly Si will grow

? ?


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Heteroepitaxy

Commensurate : atomic spacing of overlayer is sameas that of substrate

Incommensurate : atomic spacing of overlayer is not same as that of substrate due to relaxed strain

Pseudomorphic(=coherent) : 1 to 1 correspondence between atomic rowsdue to strain

relaxed strained

Lattice constant ofsubstrate

Lattice constantof film

Lattice constant ofsubstrate

Lattice constantof film

=

(coherent)


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Condition for Heteroepitaxy: examples


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Terminology

Unit cell distortion according to Poissons ratio

//

//

//

//

as

af

as

substrate

Unstrained film

af

Strained film ?

?

?

?


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FNNLFunctional Nanostructures & Nanoelectronics Lab


Calculation of misfit and strain

Ex) substrateSrTiO3: as=3.905 filmLaAlO3: af=3.79Both STO and LAOcubic structure.

Misfit, f = l 3.905-3.79 l / 3.905 = 0.029

Pseudomorphic(coherent) growth af//= as: 3.905 af:3.57

// = (3.905-3.79)/3.79=0.03+3% (+) Tensile strain

= (3.57-3.79)/3.79=-0.058-5.8% (-) Compressive strain

=3.57-3.905/ 3.79 = 0.088 (8.8%)

as

af

as

SrTiO3

LaAlO3

af

?

?

(3.905-3.79)/3.?79=0.03+3%

(3.905-3.79)/3.?79=0.03+3%

(3.905-3.79)/3.?79=0.03

+3%


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Epitaxial relationship


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Heteroepitaxy growth mode

Growth modes : depending on surface/interface energy and strain caused by mismatch

(

)/

2. > and almost no misfit

3. > and misfit

1) 2)

1. < , regardless of misfit level

1. > , large misfit


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Relaxation

Strain relaxation by misfit dislocation formation.If ||=f, totally strained, and ||=0, totally relaxed


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Epitaxial relationship

hc

Gecomposition is highereasily relaxed

*GexSi1-xfilm / Si substrate*Critical thickness VS misfit


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Heteroepitaxy system between semiconductor

Epitaxybetween dissimilar materials:* band gap (Eg) engineeringtwo different SCs could be grown alternatelymodulated Eg* Strain engineeringstrain status could be engineered using lattice mismatch

?

?


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2DEG (2 Dimensional Electron Gas)

Heteroepitaxy system between semiconductor

?


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Heterojunction (or hetero structure) devices


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Heteroepitaxy and superlattice

Superlattice: 1D periodic structure

of alternating ultrathin layers

Strained super lattice: superlatticefrom lattice mismatched layers


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Bufferlayer

Buffer layer also used for Heteroepitaxyof non-superlatticeapplications:eg) graded Si1-xGex layer

Use of buffer layer to reduce net strain accumulated

a) No buffer layer

b) With buffer layer


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Thin film deposition- Evaporation

1) thermal2) e beam

3) MBE- sputtering


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Physical vapor deposition

Deposition without chemical reaction Species to form films are physically dislodged from a source as a vapor: this vapor is

transported to substrate.

?


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Evaporation

simplest PVD thin films deposition technique using evaporation (or sublimation) of solid

source either bythermal or e-beamevaporation not the main dep. technique now for semiconductor industry (limited step coverage, less

reliability) any more, but still used for some specific purpose

?


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Source configuration for thermal evaporation

source charge

f


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Evaporation of sources

High temperature needed for some elements

E b


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E-beam source

(Some materials which need high temperature to be evaporated)

E it i l th t h i MBE ( l l b it )


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Epitaxial growth technique: MBE (molecular beam epitaxy)

MBE is a ultra high vacuum form of evaporation to produce high quality, epitaxialfilms

M l l b it


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23FNNLFunctional Nanostructures & Nanoelectronics Lab Semiconductor processing

Molecular beam epitaxy

E l S l tti b MBE


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Example: Superlattice by MBE

PVD- Evaporation

1) thermal2) e beam3) MBE

- sputtering?

S tt i


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Sputtering

Thin film deposition by energetic ion bombardment, which physically dislodge atoms from

target surface

substrate

target

S tt i


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Sputtering

10KeV

DC diode Sp ttering


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DC diode Sputtering

Magnetron Sputtering


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(Erosion ring)

(Erosion ring)



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29FNNL Semiconductor processing


Target size larger than sample, distance optimization. Heart shaped magnetron to address erosion profile: Typical modern tools

20141103 Thin Film Physics 2

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