Transcript Thin films - MyCourses

Thin films
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Thin films: different from bulk?
• Thickness: nm to µm
• Properties thickness dependent
electrical (resistivity)
optical (transmission)
mechanical (Young’s modulus)
thermal (conductivity)
• Structure: amorphous, polycrystalline or single crystalline
• Structure depends on deposition method
• Structure changes in high temperature steps
• Often severe stresses (tensile or compressive)
Thickness dependent resistivity
Thickness dependent dielectric
constant
Atomic Layer Deposited SrTiO3
Vehkamäki
Thickness dependent structure
High resolution TEM pictures
ALD deposited ZrO2: the 4
nm thick film Is amorphous.
ALD deposited ZrO2: 12 nm thick film
is polycrystalline.
from ref. Kukli 2007.
PVD: Physical Vapor Deposition
The whole wafer is
covered by the
deposited film.
Evaporation
Simple:
wafer
Heat metal until vapor pressure high
enough  metal vapor will be
transported in vacuum to the wafer.
Metal vapor condensation results in
film growth.
electron
beam gun
crucible
Very few parameters to change 
Cannot optimize film quality.
Sputtering
target
Electric field excites argon
plasma.
Accelerated argon ions hit metal
atoms from target.
Target atoms are transported in
vacuum to the wafer.
wafer
Many parameters:
power, pressure, temperature,
gas specie (Ar usually)
Metallic films usually by PVD
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conductors (Al, Au, Cu)
resistors (Ta, W, Pt)
capacitor electrodes (poly-Si, Al, Mo)
mechanical materials (Al-movable mirrors)
magnetic materials (Ni coils)
protective coatings (Cr, Ni etch masks)
optical materials (mirrors, reflectors, IR filters)
catalysts (Pt, Pd in chemical sensors)
Sheet resistance
Rs  /T
Rs is in units of Ohm, but it is usually denoted by Ohm/square
to emphasize the concept of sheet resistance. Resistance of a
conductor line can now be easily calculated by breaking down
the conductor into n squares: R = nRs
Aluminum film 1 µm thick, sheet resistrance ?
Tungsten film, 100 nm thick, sheet resistance ?
Resistor sheet resistance
Figure 2.8: Conceptualizing metal line resistance: four squares
with sheet resistance Rs in series gives resistance as R = 4Rs.
Resistor design
L
W
How to increase resistor resistance ?
1. Change L: make it longer
2. Change W: make it narrower
3. Change T: make it thinner
4. Change ρ: choose material with higher resistivity
T
PVD films for solar cells
0.5 µm
50 nm
2 µm
1 µm
Poortmans: Thin film solar cells
CVD: Chemical Vapor Deposition
gas phase convection
diffusion through
boundary layer
surface processes
(adsorption, film
deposition, desorption)
The whole wafer is covered by the deposited film.
Common CVD processes
• SiH4 (g) ==> Si (s) + 2 H2 (g)
• SiCl4 (g) + 2 H2 (g) + O2 (g) ==>
SiO2 (s) + 4 HCl (g)
• 3 SiH2Cl2 (g) + 4 NH3 (g) ==>
Si3N4 (s) + 6 H2 (g) + 6 HCl (g)
Thermal vs. CVD oxide
thermal,
1000oC
silicon
Applicable
only on
silicon.
High
temperature.
Deposition
also on
metals.
silicon
Low
temperature.
(PE)CVD
300-450oC
Plasma Enhanced CVD
Deposition can be
done at 300oC.
Thermal CVD is
usually 400-700oC
(Thermal oxidation at
1000oC)
Oxide:
SiH4 (g) + N2O (g) ==> SiO2 (s) + N2 (g) + 2H2 (g)
Nitride:
3 SiH2Cl2 (g) + 4 NH3 (g) ==> Si3N4 (s) + 6 H2 (g) + 6 HCl (g)
SiNx:H: thermal vs. plasma
Thermal CVD at 900oC
PECVD at 300oC
Smith: J.Electrochem.Soc. 137 (1990), p. 614
Dielectric films
SiO2
SiO2
SiO2
SiO2
gate oxide in CMOS
isolation oxide in CMOS
diffusion mask
etch mask in MEMS
1-50 nm
100-1000 nm
500 nm
100-1000 nm
Si3N4 oxidation mask
Si3N4 membrane in MEMS
100 nm
50-200 nm
Si3N4 capacitor dielectric
Al2O3 capacitor dielectric
HfO2 capacitor dielectric
5-20 nm
1-20 nm
1-20 nm
SiNx
500-1000 nm
passivation coating
Sample wafers !
Thin film patterns
The whole wafer is covered by the deposited film.
If you want patterns of films, you have to do lithography
and etching.
Lithography
Etching
Remove photoresist
Etching two-layer films
<Si>
If two layers are
perfectly aligned,
they were made in
the same litho &
etch steps.
<Si>
Otherwise
alignment error
would be visible.
<Si>
Two separate
layers: make
upper pattern
larger than
bottom pattern!
poly = CVD polysilicon
=polycrystalline silicon
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SiH4 (g) ==> Si (s) + 2 H2 (g)
Deposited by CVD at 625oC
Usually deposited undoped
Doping after deposition by diffusion/implantation
Annealing typically 950oC, 1 h to active dopants
Heavy doping ca. 500 µΩ-cm
Grain size ca. 200-300 nm
Annealing changes film stress (and grain size)
Typical thickness 100 nm-2 µm
ALD: Atomic Layer Deposition
Precursors introduced in pulses, with purging in-between
ALD: surface reactions
ALD films
• Al2O3
• Al2O3
• HfO2
diffusion barrier
hard mask in etching
capacitor dielectric
1-20 nm
1-20 nm
1-20 nm
• TiN
• TiN
• TaN
electrode
protective coating
barrier layer
50-100 nm
50-100 nm
1-10 nm
• Pt
catalyst
1-5 nm
Growth modes
layer-by-layer
island growth
columnar growth
Step coverage in deposition
H
A
B
Ratio of film thickness
on sidewall to horizontal
surfaces (100% =
conformal coverage)
Cote, D.R. et al: Low-temperature CVD processes and dielectrics, IBM
J.Res.Dev. 39 (1995), p. 437
ALD step coverage
Excellent conformality: deposition is a surface controlled
reaction.
Al2O3/TiO2 nanolaminate
TiN barrier
Franssila: Microfabrication
Electroplating
Typical plated metals:
-nickel (Ni)
-copper (Cu)
-gold (Au)
Not applicable to:
-aluminum (Al)
-most refractory metals
(W, Ti, ...)
Electroplated structures
a) Seed layer sputtering and lithography
b) Electroplating metal
c) Resist stripping
d) Seed layer removal
Why use electroplating ?
Metals like copper and gold do
not have anisotropic plasma
etch processes available
 If you want vertical walls,
electroplating is a solution.
Released plated metals
Stresses in thin films
The substrate is in opposite stress state !
Origin of stress
• Extrinsic stresses:
thermal expansion mismatch
Intrinsic stresses: deposition process dependent
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low energy deposition
 no energy for relaxation process
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high energy deposition
 non-equilibrium, forced positions
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impurities, voids, grain boundaries
Cantilever bending
Fang, W. & C.-Y. Lo, On the thermal expansion coefficients of thin films, Sensors &Actuators 84 (2000), p. 310
Stresses in bimetal cantilever
Generic thin film structure
surface
thin film 2
interface 2
thin film
film 11
thin
interface 1
substrate
Various interfacial
processes take
place during
following process
steps and during
device operation !
Reactions in thin films
Surface reaction:
Titanium nitride formation
2Ti + N2  2 TiN
N2
Ti
<Si>
heat
Interface reaction:
Titanium silicide formation
Si + 2 Ti  TiSi2
Interfaces
Stability of interface in subsequent processing and during use ?
Barriers and adhesion layers
In order to stabilize the interfaces, additional films are
introduced:
-to improve adhesion, e.g. Ti/Pt, Cr/Au
-to prevent interdiffusion,
Si/TiW/Al, SiO2/TaNx/Cu; SiO2/SiNx/Cu
-to prevent ion movement: glass/Al2O3/poly-Si
-to protect from ambient: SiO2/SiNx
Multilevel metallization with
Ti/TiN barriers
Ti/TiN
Ti/TiN
Al
SiNx
SiO2
W
Al
Copper for IC metallization
Barrier:
Copper seed
t < 10 nm thick
ρ < 500 µΩ-cm
Cl conc. < 2%
unif. < 2%
step coverage
>90%
rate > 3 nm/min
t > 2 nm
unif. < 2%
step coverage ~100%
rate > 10 nm/min
growth and adhesion
on etch stopper
Low-k:
General:
Etch stopper:
CMP compatible
Tdepo < 400oC
adhesion on etch
stopper
low variation
low particle generation
large process window
growth and adhesion
on dielectric
growth and adhesion
on barrier
Acoustic multilayers
Glass wafer
Al
Mo
(300 nm)
(50 nm)
ZnO
(2300 nm)
Au
(200 nm)
Ni
(50 nm)
SiO2
W
TiW
SiO2
W
TiW
(1580 nm)
(1350 nm)
(30 nm)
(1580 nm)
(1350 nm)
(30 nm)
Film characterization needs
-spatial resolution (image spot size)
-depth resolution (surface vs. bulk properties)
-elemental detection (constituents, impurities)
-structural information (grain structure)
-dimensional characterization (thickness)
-mechanical properties (curvature, stress,…)
-surface properties (roughness, reflectivity,…)
-top view vs. cross sectional imaging
-…
Sputtered TiN characterization
Thin films
• On this course, we are interested in
applications of thin films in
microfabrication
• Prof. Jari Koskinen is teaching Thin film
technology (period IV):
• “Principles of vacuum technology, surface physics
and surface-ion interactions and low pressure
plasma. Thin film methods: Physical vapor
deposition, chemical vapor deposition, and other
plasma. Characterization methods for thin films to
determine, structure, composition, and mechanical
and optical properties.”