Micro/Nanosystems Technology - Technische Fakultät

Micro / Nanosystems TechnologyWagner / Meyners 1

Micro/Nanosystems Technology

Dr. Dirk Meyners

Prof. Bernhard Wagner



Outline

• Thin film deposition techniques

– Properties of thin films

– Evaporation

– Sputtering

– Pulsed Laser Deposition

– Literature


Properties of thin films

The thickness of thin films

range between few atomic

layers (1 nanometer) and

several 100 micrometers.

Examples of extreme cases

are

• tunnel barriers in

magnetic tunnel junctions

(pressure sensor)

• microstructures fabricated

by LIGA

MgOReference CoFeB/CoFeBSi

Christoph Mitterbauer, FEI

microstructures of 800µm height micro-connector



Thin films and bulk material show different physical properties

• different structural morphology

• different density

• different electrical resistance

• different temperature dependence of electrical resistance

• ...

Thin film properties are mainly influenced by film structure including

morphology, crystallinity and grain size.

However, similar structural morphologies are found for all

material classes and different processing methods used to

fabricate thin films.



The basic processes involved in film growth are:

• adsorption: incident atoms become bonded adatoms (physisorption

or chemisorption)

• shadowing: The number of incident atoms is reduced due to the

shadowing effect of rough surfaces.

• surface diffusion: Adatoms move over the film surface until they

desorb or are trapped at low-energy lattice sites.

• bulk diffusion: By bulk diffusion the incorporated atoms reach their

equilibrium position in the lattice

• desorption

Image source: R. Zengerle, lecture notes, Mikrosystemtechnik

adsorbed

atomsdesorption

monolayer

bulk diffusionabsorbed atoms

surface diffusion

cluster



• Surface diffusion, bulk diffusion and sublimation are thermally

activated processes. Their characteristic activation energies scale

directly with the melting point TM.

• Hence the intensities of these processes are functions of

the substrate temperature TS.

• Depending on the predominant process different structural

morphologies develop.

Structure zone models

The earliest structure-zone model was proposed by Movchan and

Demchishin for evaporated films. The film structures are classified

according to three zones based on the ratio TS/TM.


The earliest structure-zone model

• occurs at TS/TM so low (<0.25) that surface diffusion is negligible

• shadowing is not compensated by surface diffusion

• At the beginning spicular crystallites are formed

• These crystallites grow to inverted conelike units capped by

domes and separated by voids and boundaries that are several

nanometers wide.

zone1:

Image source: W. Menz, Mikrosystemtechnik für Ingenieure, 2005



• occurs at 0.25 < TS/TM < 0.45. Surface diffusion is predominant.

• The structure is columnar with tighter grain boundaries

(<1nm)

• The columnar grain size increases with TS/TM in accordance with

the activation energy of surface and boundary diffusion.

zone2:




• occurs at 0.45 < TS/TM. The growth is dominated by bulk diffusion.

• The film structure is characterized by equiaxed grains.

zone3:



Structure zone model and film properties

Zone 1 Zone 2 Zone 3

TS/TM < 0.25 0.25 < TS/TM < 0.45 0.45 < TS/TM

shadowing and low

surface diffusion

high surface diffusion bulk diffusion

crystallites, void

boundaries

columnar grains,

tighter grain

boundaries

equiaxed grains

porous structure and

low density

medium density high density

low adhesion to

substrate

medium adhesion to

substrate

high adhesion to

substrate


Structure zone model for sputtered films

• proposed by Thornton

• He introduced the sputtering gas pressure as new variable and

added a fourth zone, the transition zone T between zone 1 and 2.

• The transition temperatures increase with increasing gas

pressure.

• Zone T: The influence of shadowing is partly compensated by

surface diffusion. The film structure with fibrous grains has higher

density compared to Zone 1; voids and domes are absent.

Image source:

W. Menz,

Mikrosystemtechnik

für Ingenieure, 2005


Structure zone model for sputtered films

If the pressure is increased, then the probability for collisions between the

sputtered atoms and the sputtering gas atoms increases also (s. sect. vacuum

technology).

The sputtered atoms loose kinetic energy.

After adsorption less energy is available for surface diffusion processes.

(a) The transition temperatures increase with increasing gas pressure.

(b) Zone T is less pronounced at high gas pressure.

Side note: The smaller surface diffusion leads to increased shadowing effects

connected with poor edge coverage and low film adhesion. On the other

hand, at high pressures the oblique component of the deposition flux is

increased because of gas scattering. This improves edge coverage again.

However, best coverage is achieved with high surface mobility of the adatoms.

a: ideal case; high surface diffusion leads to homogeneous

edge coverage

b: inhomogeneous coverage due to shadowing and low

surface diffusion


Structure zone model and film properties

Evaporants TM[K] Transition to

zone 2

TS[K]

zone 3

TS[K]

Al 933 > 233 > 420

Au 1 336 > 344 > 600

Ti 1 998 > 500 > 899

Pt 2 042 > 510 > 919

Cr 2 148 > 537 > 967

W 3 695 > 913 > 1 644

Zone 2 and 3 of evaporants with high melting temperatures cannot be reached.



Due to the same reasons a growing film can also

expand.

compressive stress

affects adhesion

Since internal stresses tend to increase with

thickness, promoting film peeling, they are a

prime limitation to growth of very thick films.

• Deposition at elevated temperatures

• Cooling down after deposition

• Differences in thermal expansion coefficients

between film and substrate

mechanical stress.

• Growing films can shrink relative to the substrate due

to surface tension forces or misfit epitaxial growth.

tensile stress

affects adhesion

- film - substrate

- film - substrate



• Thermal expansion coefficients

Data source:

S. Kalpakjian, S. R. Schmid, E. Werner:

Werkstofftechnik, Pearson, 2011

Material Α [µm/m 1/°C]

Al 23.6

Au 19.3

Ti 8.35

W 4,5

Si 7.63


Measurement of film stress

Often substrates show curvature.

Measure profile of substrate before

deposition and substrate/film profile

afterwards.

Differential measurement

Stoney equation:

f

S

S

Sf

d

d

v

Y

R

2

16

1

film or substrate : subskript

thickness :

ratio Poisson :

modulus s Young':

curvature of radius :

stress :

fsi

d

Y

R

i

i

i

i

R

Data source: L. Zhang et al., Microstructure, residual stress, and fracture

of sputtered TiN films, Surface & Coatings Technology 224 (2013) 120–125

Sf dd


The Stoney equation

Fundamentals:

modulus s Young':

Law sHook' :elongationelastic

:strain

elongation after length

:ntdisplaceme

YYA

F

Y

dx

du

dx

dxdudxx

dudxuxduudxx

xuu

11

:


The Stoney equation

Fundamentals:

ratio sPoisson' :

:ncontractio ltransversa

x vY

vv xy

vY

G

GG

12

:materialsisotropic

modulus shear :

:strain shearing

xy

xy


The Stoney equation

Fundamentals:

yxzz

xzyy

zyxx

vY

vY

vY

1

1

1

:dim. 3 inlaw sHook'

stress components + Hook’s law strain components

integral over strained compound displacements

z

w

y

vx

u

z

y

x


The Stoney equation

Fundamentals:

Force: translatory motion

Moment: rotational motion

Couple of forces (F1 = -F2):

FrMA

1

2211

aFM

FrFrM

O

O

Application of force F is equivalent

to application of parallel displaced

force F plus moment M.


The Stoney equation

Starting point:

In mechanical equilibrium, net force

and bending moment vanish on any

film/substrate cross section:

0dAF

0dAyM

Image source: M. Ohring, Materials Science of thin films,

2nd edition, Academic Press, San Diego, 2002

eq. 1


The Stoney equation

Each interfacial set of forces can

be replaced by an equivalent

combination of force and moment:

SfSSff FFMFMF and , ; ,

Image source: M. Ohring, Materials Science of thin films,

2nd edition, Academic Press, San Diego, 2002

w

eq. 1

Sf

fSfMM

Fdd

2

Isolated beam bent by M linear stress distribution

R

YdY mm

2

strain of the outer/inner fiber:

R

d

R

RdRm

2

2

Hook’s law:

eq. 2


The Stoney equation

Stress in fiber with distance y from neutral plane:

2

0

2

22

2)(

d

m

d

d

dywyd

ydAyyM

Calculation of bending moment across beam section:

2)(

d

yy m

R

wYdwddyy

dw m

d

m126

14

322

0

2

Extension to the film/substrate system:

R

wdYM

R

wdYM SS

S

ff

f1212

33

and


The Stoney equation

eq. 2

Since

R

wdY

R

wdYFddSSfffSf

12122

33

: Sf dd

R

wdYF

d SSf

S

122

3

f

SS

f

f

fRd

dY

wd

F

6

2

Stoney equation for

1-dimensional bending

(uniaxial stress)

Stoney equation for

biaxial-stress distribution f

SS

f

f

fdR

dY

wd

F

16

2

For extension to elastically anisotropic materials (such as Si(001) wafers) see:

Janssen, G. C. A. M., et al. "Celebrating the 100th anniversary of the Stoney

equation for film stress: Developments from polycrystalline steel strips to single

crystal silicon wafers." Thin Solid Films 517.6 (2009): 1858-1867.



Outline



– Evaporation

– Sputtering


– Literature


Evaporation

Image source: http://corvus.ett.bme.hu

Basic operation:

1. thermal evaporation

(1000 – 3500°C)

2. transport of particles to

substrate

3. condensation of particles

on substrate (cooling or

heating to 100 – 600°C)


Evaporation1. Thermal evaporation: sources

“dimpled boat”

inductive coil

e-gun

electron beam

evaporation



Evaporation1. Thermal evaporation:

“dimpled boat”

I

• made from refractory metals (Ta, W)

• heated by passing current of the

order of I = 100 A through it

• evaporation rate Q depends strongly

on temperature

pressure vapor mequilibriu :, 2

)(s

s PMRT

TPQ

Knudsen equation:

nevaporatio of

heat molar :, exp0 ee

s HRT

HPP



Precise temperature control is needed for constant evaporation rate.

Evaporation of alloys is difficult. Initially the component with higher

vapor pressure is evaporated. This leads to a composition variation

across the cross section of the deposited film.

Use multiple sources (one source for each component)

Image source:

S. Büttgenbach,

Mikromechanik, Teubner

Studienbücher, 1991



inductive coil• An alternating current flows through

the coil generating a high frequency

electromagnetic field.

• Induced currents in the conductive

crucible heat the evaporant

• If the evaporant is conductive, then

insulating crucibles (with higher TM)

can be used.crucible

• Dimpled boat and inductively heated sources suffer from

contamination problems due to the direct contact of the melt to

the crucible.



Advantages:

• no contamination of evaporant

• high temperatures

• high evaporation rates

e-gun

electron beammagnetic field

water cooling

locally melted

evaporant

Disadvantages:

• deflection and deceleration of electrons cause X-ray radiation which can damage the substrate or growing film

• complex setup


Evaporation2. Transport of particles:

0J

B

0r

P

R cos0JJ

source

substrate

Jθ denotes the evaporant flux at radius r0 from the source and at angle θ

sphere:

disc: cosine flux distribution cos0JJ

2

00 4/ rQJJ



0J

B

sin0r

P

R

cos0JJ

source

substrate

2

0

0r

QJ

0

2

0

2

0

000

2

0

2

0

00 sin2cossin JrdrrJdrdrJQ

0r

J

0J

or equation 1

0r

dr sin0 d



0J

B

sin0r

P

R cos0JJ

source

substrate

2

0

0r

QJ

0r

J

0J

or equation 1

dr0

0r

0

2

0

2

0

000

2

0

2

0

00 sin2cossin JrdrrJdrdrJQ



0J

B

P

R

SJ

source

3

0 cosJJS cos/0rr

0r

J

Flux at point S: equation 1 + cosine flux distr.

Deposition rate is determined by flux

perpendicular to the substrate at point S:

SJ

r

2

0

44

0

coscoscos

r

QJJJ S



4cos

Example: substrate radius is one-fourth of substrate to source distance r0

- term 10 % reduction in J┴

There is always a trade-off between non-uniformity at short distances and

evaporant waste at large distances.

2

0

4cos

r

QJ


Evaporation3. Condensation of particles :

structure zone model

At large distances r0

J1n

2n

0

2

1

2

1

2

1

cos

cos

Jn

Jn

t

t

Inhomogeneous coverage at low surface diffusion

2Image source:

M. Madou,

Fundamentals of

Microfabrication,

2002


Thin Film technique 2

High Vavuum Evaporating-System PLS 500

• System: Electron-beam evaporator

• Substrate: Dimension: up to 100mm

Substrate heater up to 600°C

• Film-thickness monitoring with oscillating quartz



Outline



– Evaporation

– Sputtering


– Literature


Sputtering


massflow controllerto vacuum pump

power

supply

argon ion

metal atom

cathode

target

plasma

substrate

anode

cooling/

heating


Sputtering

Basic operation (DC-Sputtering):

1. In an inert gas plasma region

atoms are ionized (argon ions).

2. These ions are accelerated by

an electrical field towards the

cathode.

3. The target material is bombarded by the ions. It is sputtered away

mainly as neutral atoms by momentum transfer.

4. On their path to the substrate the target atoms hit with sputter gas

atoms and lose energy. Thereby the oblique component of the

deposition flux is increased.

5. Condensation of particles on substrate placed on the anode (cooling

or heating).


Sputtering

• A plasma is sometimes referred to as the fourth state of aggregation.

• Components:

– electron gas

– ion gas

– neutral gas

• In glow discharges the ratio between ionized and neutral gas species is on the order of 10-6 to 10-4.

• Ionization energy of argon atoms: 15.7 eV

• Methods of plasma generation:

– thermal excitation (nuclear fusion)

– radiation

– electrostatic fields

– electromagnetic fields

Image source: R. Zengerle, lecture

notes, Mikrosystemtechnik


Sputtering

Use of plasmas in MST:

– (Magnetron-) Sputter Deposition

Substrate ≠ Target

– Dry etching

Substrate = Target

– Plasma Enhanced Chemical Vapor Deposition


Glow discharge

• Simplest plasma reactor: opposed parallel-plate electrodes in chamber at low Argon pressure (10-3 to 10 mbar)

• The applied voltage determines the energy of electrons and ions.

• Electrical breakdown occurs when accelerated electrons transfer kinetic energy > 15.7eV to argon neutrals.

second free electron and a positive ion.

both electrons reenergize

creation of an avalanche of ions and electrons

d

emission of characteristic glow (blue in case of argon)


Glow discharge

• glow region = good conductor low

electrical field (Vp≈const.)

• potential drops in front of the

electrodes where electrical double

layers are formed (plasma sheath)

• In the Crookes dark space ions are

accelerated towards the negative

cathode emission of secondary

electrons plasma is sustained Crookes dark space Anode dark space

• Vp: plasma loses electrons to the

walls, thereby acquiring a positive charge

4

and , ,,

,

ioneione

ioneione

vnJvv

• Ve: electrons near cathode rapidly accelerate away; ions, being more

massive, accelerate towards the cathode more slowly greater ion

concentration in front of cathode large field

Image source: M. Madou, Fundamentals of Microfabrication, 2002


Paschen’s law

• The voltage for electrical breakdown depends on the geometry of the electrode gap, the gas, and the pressure.

• F. Paschen: V = f (P x d)

d: gap distance

• Minimum breakdown voltage

• Sharp rise for low P x d: Electrode space is too small for ionization to occur

• Some electrostatic micromachines (motors, switches) work on the low P x d side.

• Rise for increasing P x d: Collisions become more frequent a higher voltage is needed to accelerate the electrons to a sufficient kinetic energy .


bre

akdow

n v

oltage

Gas Vmin [V] P x d [Torr cm]

Air 327 0.567

(1atmd=8µm)

Ar 137 0.9

N2 251 0.67

O2 450 0.7


RF-Sputtering

argon

plasma

to vacuum pump

capacitor

RF-generator

cathode

target

anode

reactive gas

molecules

substrate• parallel plate reactor

• anode is grounded

• cathode is coupled via a capacitor to the RF-generator (13.56 MHz no interference to radio-transmitted signals)

• target on cathode

• substrate on anode

• RF-field electrons oscillate and collide with gas molecules

sustainable plasma without secondary electron emission from the cathode

• Advantages:

– lower gas pressure

– sputtering of insulators possible

• Paschen behavior: V = f (P x λe) , λe: mean free path of electrons

Image source:

W. Menz,

Mikrosystemtechnik

für Ingenieure, 2005


RF-Sputtering

• time-averaged potential distribution is similar to

DC-plasma

• VP: initial electron loss plasma potential

• The cathode automatically develop a negative

dc bias (self-bias VDC):

after plasma initiation electrons

charge up the electrode, no charge can be

transferred over the capacitor cathode

retains a negative dc bias

• typical VDC ≈ 300V cathode is bombarded with 300eV ions etching or

sputtering


time-averaged

potential distribution

ione vv


RF-Sputtering

• The time-average of plasma potential VP, self

bias potential VDC and peak-to-peak RF

voltage VRF,pp are related as:

• Maximum energy of ions striking the cathode:

• Surface damage and ion implantation of the substrate have to be avoided

has to be minimized (typically 20eV)

• Considering the geometry of parallel plate reactors one can derive:

AP > AT


time-dependent potentials

DC

ppRF

P VV

V 2

2,

TPDCcathode eVVVeE max,

Panode eVE max,

area odeanode/cath :A , TP,

2

T

P

P

T

A

A

V

V

ground the entire sputtering chamber


Magnetron Sputtering

• A crosswise magnetic field below the cathode traps electrons in orbits.

electron path length in the plasma increases

higher degree of ionization

gas pressure can be reduced by one magnitude down to 10-3 mbar

sputtered particles retain their kinetic energy upon reaching substrate

(beneficial for film structure)

massflow controllerto vacuum pump

argon ion

metal atom

cathode

target

plasma

anode

cooling/

heating

substrate

substrate

bias

power

supplycooling

magnet



Sputtering of Alloys

Amount W of sputtered material:

distance cathode toanode:

pressurechamber :

current discharge:

voltageoperating:

constantality proportion:

d

P

I

V

k

Pd

kVIW

Sputtering yield Y:

energy binding surface:

incidence of angle:

particel sputtered of mass:

ionincident of mass:

ionincident ofenergy :

,,,2

1

2

1

U

M

M

E

UM

MEYY


Sputtering of Alloys

Sputtering Yield Y for multiple-component materials:

iYYY ii component of yield sputtering: i

Sputtering yields and surface composition of a binary component:

state)(steady t bombardmenion after

component ofion concentrat surface:

tbombardmenion before

component ofion concentrat surface:

*

1

2

2

1

2

1

ic

ic c

c

c

c

Y

Y

i

i*

*


Thin Film technique 1

Sputter Deposition: Von Ardenne CS 730 S

• Cluster deposition system with 3 chambers and 9 sputter sources

• 8“ and 4“ targets

• Load-lock with 10 x 6“ substrate plate magazine

• RF and DC sputtering up to 3 kW

• 6“ Substrates, radiative heating possible

• Sputter gases: Ar, N2, O2

• Magnetic bias field (100 Oe) possible

• RF etching

Current target list:

• Metals: Ag, Au, Cu, Al, Cr, Ru, Ta, Ti, Mo

• Alloys: FeCo, FeCoBSi, TbFe, NiTi, NiFe, CoFeB, FeAl,

FeB, FeHf, FePd, MnIr, TiNiCu

• Oxides, Nitrides: Al2O3, MgO, Ni3N8, ZnO2

(other oxides by reactive sputtering)


Comparison of evaporation with sputtering

Evaporation Sputtering

Rate 1000 atomic layers /s 1 atomic layer /s

magnetron:

10 atomic layers /s

Choice of Materials Limited Almost unlimited

Purity Better (no gas

inclusions)

Possibility of

incorporating sputter

gas molecules

Surface damage Very low,

X-ray damage (e-beam

evaporation)

Damage due to ionic

bombardment

Alloy compositions Little control (multiple

sources)

Good control

Particle energy 0.2 eV (thermal) 4 .. 40 eV

Adhesion Often poor Excellent

Shadowing Large Small

Equipment Low cost More expensive



Outline



– Evaporation

– Sputtering


– Literature


Pulsed Laser Deposition

• High power laser pulses

(typically ~108 W/cm2) melt,

evaporate and ionize

material from the target

surface (laser ablation).

• Upon ablation a luminous

plasma plume expands

rapidly from target surface.

(velocity ~ 106 cm/s)

• The ablated material

condenses on an

appropriately placed

substrate.

Image source: superconductivity.et.anl.gov/Techniques/PLD.html


Pulsed Laser DepositionAdvantages:

• Capability for stoichiometric transfer of

material from target to substrate

• Very good thickness control by counting laser

pulses (deposition rate: few atomic layers /s)

• Clean process without filaments and crucibles

• Deposition can occur in inert and reactive

background gases.

Disadvantages:

• The plasma plume is highly forward directed non-uniform thickness and composition

variation across the film

Rasterizing the laser spot across the target surface and moving/rotating the substrate

• The plume contains globules of molten material with up to 10µm diameter .



Image source: Pulsed Laser Deposition of Thin Films, Chrisey & Hubler, Wiley

Using mechanical velocity filters (e.g. a multiple-fin rotor operated at 3000 rpm)



Film growth can be influenced by

• laser fluence (number of impinging photons

per cm2)

• background gas pressure (collisions slow

down high energetic particles resputtering

the film already deposited on the substrate is

avoided)

• substrate temperature (structure zone model)

Plasma plume formation:

• The fundamental processes are not completely understood.

• Laser pulse penetrates into the surface a few nanometers depending on laser

wavelength and the index of refraction of the target material.

• Electrons are removed from bulk material by strong electrical fields in the laser beam.

• These free electrons oscillate within the electromagnetic field of the laser light. collide

with atoms target is heated up locally the material is vaporized (T~10 000K)



Plasma plume investigations by

Laser-Induced-Fluorescense (LIF)

Mapping of neutral and ionic components

within slices of the plume

LIF image of ground state, singly ionized Titanium

species expanding along the target normal into

vacuum.

The image was taken 3µs after the laser pulse

struck the target surface.

Image source: Physics department, Queen’s University,

Belfast



a: Laser pulse is absorbed, melting and vaporization begin.

(arrows indicate motion of solid-liquid interface)

b: Melt front propagates into the solid, accompanied by vaporization.

c: Melt front recedes (cross hatched area = resolidified material).

d: Solidification is complete, frozen capillary waves alter surface topography

The next laser pulse will interact with some / all of the resolidified material.


Thermal cycle induced by laser pulse



Laser-induced periodic surface structure

produced on an aluminum mirror with a

single 248nm pulse at 0.3 J/cm².

The ripple spacing is approximately 1µm.


Scanning electron micrographs of cone

structures produced at 5.6 J/cm².

(Scale marker is for the left half of the

image.)



• Cone formation is also observed after ion bombardment of surfaces.

• Recent experiments suggest that vaporization-resistant impurities are responsible for

laser-cone formation.

• Surface modification of the target does not appear to be an obstacle for stoichiometric

film growth (laser preconditioning of the target).

• But surface modification clearly affects film deposition rate. A decay of rate with

cumulative exposure occurs in nearly every material.


Deposition rate for YBCO exposed

at 308nm and 3.3 J/cm².

The deposition rate in oxygen

atmosphere is smaller due to

scattering and broadening of the

plume.



Outline



– Evaporation

– Sputtering


– Literature


Literature

• Donald Smith, Thin-Film Deposition - Principles & Practice,

McGraw-Hill, 1995

• Milton Ohring, Material Science of Thin Films – Deposition and

Structure, 2nd edition, Academic Press, 2002

• Marc Madou, Fundamentals of Microfabrication, CRC Press, 2002

• Stephanus Büttgenbach, Mikromechanik, Teubner, 1991

• Douglas Chrisey, Graham Hubler, ed., Pulsed Laser Deposition of

Thin Films, Wiley & Sons, 1994

Documents

Micro/Nanosystems Technology - Technische Fakultät