Transcript Electrical and optical

Electrical and optical
properties of thin films
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Outline
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Metallic (conductive) films
Contact resistance
Low dielectric constant films
High dielectric constant films
Optics in thin films
Metallic mirrors
Dielectric mirrors
Other optical components
Resistivity
Resistivity (2)
ρ = ρresidual + ρtemp
Linear TCR above
Debye temperature
(typically 200-400K)
Murarka: Metallization
Resistivity: alloying effects
Murarka
Annealing defects away
Annealing defects away at
elevated temperature
lowers resistance (no
reaction with underlying
film/substrate)
Murarka: Metallization
Thickness dependent resistivity
Linewidth dependent resistivity
Cu
ITRS 2007
Resistivity as a function of film
thickness for wide lines
γ = film thickness/mean free path
Mean free paths typically tens of nanometers at RT
Murarka
Resistivity in polycrystalline films
R = reflectivity at grain boundaries (0.17 for Al, 0.24 for copper)
lo = mean free path inside grain
d = spacing between reflecting planes
Grain boundaries trap impurities, and above
solubility limit, this leads to segregation
Murarka
Resistivity depends on patterns!
You cannot calculate
thickness from
resistance
R = ρL/Wt
because thin film
resistivity ρ is
linewidth and thickness
dependent.
G.B. Alers, J. Sukamto, S. Park, G. Harm and J. Reid, Novellus Systems, San Jose -- Semiconductor International, 5/1/2006
Grain size affected by:
-underlying film (chemistry and texture)
-deposition process (sputtering vs. plating; & plating A vs. plating B)
-material purity
-thermal treatments
-geometry of structures on wafer
G.B. Alers, J. Sukamto, S. Park, G. Harm and J. Reid, Novellus Systems, San Jose -- Semiconductor International, 5/1/2006
Resistivity: impurity effects
Murarka
Specific contact resistance, Rc
Dopant level of
semiconductor must be
high, but the depth of
doped region need not
be be thick.
e.g. ion implantation
dose 1E15 cm-2, 20 keV,
is ca. 100 nm deep, so
concentration
is >> 10E20 cm-2
Ohring
Specific contact resistance (2)
Ti reduces any SiO2 at
the interface to TiO
 rc down
TiN is high resistivity
material  higher rc
CuTi starts to form
above 300oC
TiN is a better barrier
and rc is reduced the
higher the anneal
temperature
Low-k dielectrics
R
C
L
HW
WL
T
  RCL
2
Scaling of RC time delay
W
 L
C'  n  C
T
n
R' 
W
T
L
n R
2
  H  W  
    
  n  n  
 ' n RC
2
H
L
metal
dielectric
Dielectric constant
CVD SiO2-based glasses with r  4
Fluorine doped oxide by CVD r 3.6
Carbon doping, with CH3-groups in silicon
dioxide, designated as SiOC:H, r  2.7
SiOC:H
Composition of SiOC:H films is typically 20-25 at% Si, 30-40% O,
15 % C, and 20-40% hydrogen.
SiOC:H
Characterization needs for
new dielectrics
Parameter
-CMP rate
-Tg/Td
-plasma resistance
-cleaning resistance
-shrinkage
-adhesion
-outgassing
-porosity
-pore size
-shelf life
-viscosity
-impurities
-CTE
-loss tangent
Comment
-Young’s modulus 1-10 GPa, high polish rates
-glass transition/decomposition temperatures (ca. 450C)
-organic materials are etched in oxygen plasma
-photoresist removers and solvents
-volume changes upon heat treatment as solvents evaporate
-Scotch tape test is the first hurdle
-even cured films may release gases into sputtering vacuum
-tightly controlled for reproducible 
-too big pores behave like pinholes
-decomposition during storage not unlike photoresists
-film thickness depends on viscocity (and spin speed)
-(alkali) metals have to be measured
-thermal expansion of polymers highly variable
-electrical losses at high frequencies must be understood
High dielectric constant films
MOS capacitance:
C = εA/d
Make ε high, capacitance
increases
HfZrO high-k dielectric
IL = interlayer of SiO2
Gallegos, Triyoso, Raymond, MEE 2008
(the alternative ways are
coming to trouble: making
capacitor area A smaller is
expensive; and making
thickness d smaller is
approaching atomic
dimensions.
Equivalent oxide thickness EOT
 SiO2
EOT 
 t highk  t SiO2
 highk
EOT of 5 nm hafnia with different
interfacial oxide thicknesses:
0 nm SiO2
EOT =0.8 nm
1 nm SiO2
EOT = 1.8 nm
2 nm SiO2
EOT = 2.8 nm
Al2O3 quality
Wet etch rate
as a proxy of
density and
quality.
Electrochemical and Solid-State Letters, 7 ~8! F45-F48 ~2004
Breakdown field
Electrochemical and Solid-State Letters, 7 ~8! F45-F48 ~2004
C-V measurement
Electrochemical and Solid-State Letters, 7 ~8! F45-F48 ~2004
Dielectric constant
Electrochemical and Solid-State Letters, 7 ~8! F45-F48 ~2004
Optical thin films,
general requirements
Environmental
stability
Reflection
Mechanical scratch
resistance
Waveguiding
requires
large
nhigh-nlow
Transmission,
absorption
Optical constants
N = n –ik
N = complex index of refraction
n = real index of refraction
k = index of absorption = extinction coefficient
n and k are wavelength dependent.
Intensity attenuation
I = I0 exp-αx ,absorption coefficient α = 4πk/λ
Deposition vs. optical constants
Solid line: bulk rutile TiO2
Dashed line: wet deposition
Spherical: ion assisted
deposition
Squares: evaporation
Triangles: sputtering
Ohring p. 523
Refractive
index
N = n –ik
Film n is
usually
smaller than
bulk n,
film k is
usually
larger than
bulk k.
Ohring
Thin film optics
R+T=1
Ohring
for non-absorbing films
Martin
Reflectivity & transmittance
Non-absorbing films (e.g. SiO2, Si3N4, TiO2,…)
For absorbing films (metals, silicon, …)
Thin film optics (2)
R + T + A + S =1
Reflection
Transmission
Absorption
Scattering
Absorption
R+T+S+A=1
For a solar cell or solar
thermal collector,
hopefully
R=0
T=0
S=0
A =1
Poortmans p. 182
Optical devices
Ohring p. 532
ARC: Anti Reflective Coatings
Matching: n1 = √n0·n2
n1·d = λ/4 (and 3λ/4, 5λ/4,…)
Glass n2 = 1.52, air n0=1
n1 = √1.52 = 1.23  no such material !
Silicon n2 = 3.8, air n0=1  n1 = 1.95
SiNx is almost perfectly matched !
ARC on glass
Bare glass (n=1.52)
 reflectivity 0.0423
= 4.2%
Use MgF2 (n= 1.38)
as ARC.
For 550 nm light, λ/4n
= 99.6 nm
S=single layer ARC, D=double
R = 1.26%
Ohring p. 529
Metallic mirrors
Ohring p. 512
Martin p. 293
Metallic mirror issues
Below 10 nm metals discontinouos, and optical constants ill-defined.
Reactive metals like aluminum incorporate impurities from vacuum
Reflectance reduced.
Noble metals like Pt or Ph are not affected by vacuum impurities or
ambient gases during operation.
High deposition temperature leads to grain growth and surface
roughening  scattering increases.
Aluminum surface is oxidizied during use.
Silver surface reacts with sulphur.
Noble metal adhesion is often poor.
Dielectric mirrors,
enhanced metal mirrors
SH(LH)n =
substrate, high refractive index film,
low+high film stack of n-layer pairs
nHtH = λ/4 condition for thicknesses
Film H
Film L
Film H
Film L
Film H
Substrate
Martin p. 303
Enhanced aluminum mirror
SM(LH)2 design
Glass/Aluminum/SiO2/Ta2O5/SiO2/Ta2O5
510 nm centre wavelength
Martin p. 301
(HL) multilayer filters
Martin, p. 304
Cold mirror
Visible light is
reflected
Infrared radiation passes thru
the cold mirror
Si/Ge for IR
transmittance
+dielectric
multilayer for
visible
reflection, i.e.
SM(HL)n
Ohring p. 536
λ/4 vs. rugate filters
λ/4 filter
rugate filter
rugate filter
with
apodization
Rugate filters (2)
Refractive index profile
Nitrous oxide flow rate
On glass substrate
On polycarbonate substrate