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Chapter 3
ANODIC ALUMINA MIM
CAPACITORS
3.1 Introduction
Al2O3 is one of the attractive dielectric materials with wide bandgap of 8.3eV and dielectric
constant of 8 to 10. Al2O3 MOS structure using ALD shows more than 10 years of life time
at low voltage operation [Lee et al., 2000] with high breakdown field of 30MV/cm
[H. C. Lin and Ye, 2005]. Dielectric properties of Al2O3, such as leakage, dielectric
relaxation and reliability, have been investigated by K. Allers et al [Allers et al., 2003]. In
which, Al2O3 shows more reliable and optimum performance compared to SiO2 and Ta2O5,
with low leakage current density [Allers et al., 2003]. ALD and thermal evaporation
techniques were successfully demonstrated for Al2O3 MIM capacitors [Chen et al., 2002,
Miao et al., 2009]. Recently, porous anodization also have been used in fabrication of MIM
capacitor which results a high capacitance density of 5 f F/µm2 [Hourdakis and
Nassiopoulou, 2010]. However it shows ∼40% reduction in capacitance value in the
frequency range of 1KHz to 1MHz. Moreover, the capacitance-voltage variation
coefficients are highly sensitive with frequency and temperature. This is the sign of thermal
and frequency instability due to charge traps available at the metal-insulator interface.
The barrier type anodic oxide is a better solution to improve the capacitor performance
35
because of its low defects [Huang and Hwu, 2003]. The barrier type oxide can be obtained if
the anodic oxide is insoluble in the electrolyte during anodization [Diggle et al., 1969]. But
a detail study including voltage linearity, frequency dependence of capacitance, polarization
and leakage current mechanisms on barrier type anodic oxide MIM capacitor is required but
not yet done. In this chapter, the performance and fabrication of barrier type alumina MIM
capacitor using anodic oxidation are presented in detail.
3.2 Fabrication process flow
Barrier type anodic γ-Al2O3 was obtained using various aqueous electrolytes, such as
ammonium pentaborate dissolved in H2O (bor-H2O), sulfuric acid, ammonium pentaborate
dissolved in ethyline glycol (bor-gly) and citric acid by many authors [Hickmott, 2007,
Raymond and Das, 1976, Hourdakis and Nassiopoulou, 2012]. It was observed that bor-gly
solution results low leakage and high effective barrier height compared to bor-H2O
[Hickmott, 2007]. Recently Sato et al have studied the effect of electrolyte on the
crystalline properties of anodic alumina [Yoshiteru Sato and Ono, 2010]. The bor-gly
solution shows higher crystalline count than that of other electrolytes. In this work, we have
fabricated the MIM capacitor with anodization on bor-gly electrolytes based on suitable
approach.
SiO2
Al Top electrode
Al Bottom
electrode
Anodized region
Figure 3.1: Schematic diagram of cross section of resulting MIM capacitor
36
0
20
40
60
80
100
120
140
160
5 10 15 20 25 30
Oxid
e t
hic
kn
ess
(n
m)
Anodization voltage (volts)
TA=1min
TA=2min
TA=3min
Rate = ~1.41nm/volt
0
50
100
150
200
250
5 10 15 20 25 30
Oxid
e t
hic
kn
ess
(n
m)
Anodization voltage (volts)
TA=1min
TA=2min
TA=3min
Rate = ~2.14nm/volt/min
(a) (b)
Figure 3.2: Measured thickness of anodic Al2O3 thin film for various VA and TA at differentcurrent densities
An Al (99.99% pure) thin film of 300nm was deposited on a 100nm wet oxidized SiO2
over Si (100) substrate by thermal deposition using tungsten filament at pressure of
2.5× 10−5 Torr. At 0oC, surrounded by ice bath, the Al film was anodized in a solution of
ammonium pentaborate (APB) dissolved in ethylene glycol (20gl-1) by platinum cathode of
equal size as anode, in a constant current density of 0.5mA/cm2. The solution was prepared
by adding 17gm of APB (99% pure) for every 100ml of ethylene glycol [Raymond and Das,
1976]. To avoid the etching for bottom electrode, three quarters of sample area was dipped
in the electrolyte for anodization time TA at constant voltage of VA. Once cleaned
thoroughly by deionized water, the 50nm thick Al top electrode was deposited using
thermal deposition with the shadow mask area of ∼0.6mm2. Schematic diagram of cross
section of resulting MIM capacitor is shown in Fig. 3. 1.
3.3 Structural and Electrical properties
3.3.1 Formation and Crystalline properties
Anodization was performed for various anodization voltages (VA) over anodization time
(TA). The thickness of dielectric layer was measured using elipsometry test. The measured
thickness of anodic Al2O3 thin film for various VA and TA for different anodization current
37
Al2O3
Al
SiO2
Si
100nm
(a)
Al2O3
Al
SiO2
Si
(b)
Figure 3.3: SEM images of anodized samples (a) cross sectional view and surface of sampleanodized at 30V for 1min at 0.5mA/cm2, (b) cross sectional view and surface of sampleanodized at 30V for 1min at 1mA/cm2,
40 43 47 50 53 57 60 63 67
Inte
nsi
ty
(2θ)
γ-Al2O3
γ-Al2O3
Figure 3.4: X-ray diffraction (XRD) spectra of anodized structure at VA = 30V
density is shown in Fig. 3. 2. Rate of growth of anodic Al2O3 was found as 1.4nm/V per
minute for the current density of 0.5mA/cm2. Fig. 3. 2 (b) shows the similar measurement of
thickness is done for anodization current density of 1mA/cm2. It has been observed that the
growth rate is increased to 2.1nm/V per minute. Fig. 3. 3 (a) shows the SEM cross sectional
view of anodization region and surface of various samples. The surface profile of anodized
region is shown in Fig. 3. 3 (b) which confirms ’non-porous’ or ’barrier type’ anodic Al2O3.
Fig. 3. 4 shows the X-ray diffraction (XRD) spectra as a function of scattering angle
(2θ ) of sample anodized at VA = 30V at current density of 0.5mA/cm2. The crystalline
38
0
2
4
6
8
10
-3 -2 -1 1 2 3
Ca
pa
cit
an
ce d
en
sity
(fF
/μm
2)
Voltage (V)
49.5nm28.8nm14.3nm
Figure 3.5: Measured C-V characteristics of MIM capacitor with various thicknesses
peaks at 46.2o and 67.7o are observed which confirms that the formed oxide is γ-Al2O3. The
delamination or removal of oxide layer from metal is observed at higher voltage (> 40V )
which affects the surface of dielectric layer and later deposition of top electrode.
3.3.2 Capacitance and Voltage linearity
The capacitance and leakage current have been measured using HP4155C semiconductor
parameter analyzer. Fig. 3. 5 shows measured C-V characteristics of MIM capacitor for
various dielectric thicknesses. It is observed that the stability of capacitance with voltage
increases with thickness. Measured capacitance at applied bias voltages 2V and 5V as a
function of AV is shown in Fig. 3. 6. It is observed that the linear relation between
capacitance and thickness, C = ε0εrA/d, is valid up to 25nm. The boron ions near top
electrode interface stabilize the amorphous region at higher thickness which reduces the
effective dielectric constant of anodic Al2O3.
Variations due to the voltage and temperature were estimated by calculating the voltage
coefficient of capacitance (VCC) and temperature coefficient of capacitance (TCC) [Onge
et al., 1992].
VCC =
[C(V )−C0
C0
]×106 (3.3.1)
39
1
3
5
7
9
11
5 10 15 20 25 30C
ap
aci
tan
ce d
ensi
ty (
fF/μ
m2)
Anodization voltage (volts)
V=2V
V=5V
Figure 3.6: Measured capacitance at applied bias of 2V and 5V for function of thicknesses
0
100
200
300
400
500
600
700
800
900
-3 -2 -1 1 2 3
VC
C (
pp
m/V
)
Voltage (V)
14.3nm28.8nm49.5nm
(a)
0
100
200
300
400
500
600
700
14.3 28.8 49.5
α , β
Thickness (nm)
α
β
(b)
Figure 3.7: Calculated VCC for different applied voltages for various MIM capacitors andEstimated α (ppm/V 2) and β (ppm/V ) for various thicknesses
TCC =
[C(T )−C0
C0
]×106 (3.3.2)
Fig. 3. 7 (a) shows the VCC at different applied voltages for various MIM capacitors.
It is found that the VCC values are lower at higher thicknesses. The linear and quadratic
coefficients of capacitance are calculated by fitting the following equation with measured
capacitance.
C(V ) =C0(αV 2 +βV +1
)(3.3.3)
Fig. 3. 7 (b) shows the calculated α (ppm/V 2) and β (ppm/V ) for various thicknesses
at 100KHz and 1MHz. It shows that the value of α decreases from 605ppm/V 2 to
102ppm/V 2 as thickness increases, however β does not change significantly. Fig. 3. 8
40
5
6
7
25 75 125
Cap
aci
tan
ce d
ensi
ty(f
F/μ
m2)
Temperature (oC)
1MHz
100KHz
Figure 3.8: Temperature dependence of capacitance at 100 KHz and 1 MHz.
shows the temperature dependence of capacitance at 100 KHz and 1MHz frequencies for
sample anodized at 30V. TCC value varies from 100ppm/oC to 150ppm/oC which meet
the requirement of ITRS .
3.3.3 Frequency dependence of capacitance
Fig. 3. 9 shows the frequency dependence of capacitance of Al-6 at 25oC and 125oC. The
results show that the sensitivity to frequency variation is low compared to the earlier reports
at 25oC [Chen et al., 2002, Hourdakis and Nassiopoulou, 2010]. The stable nature of
capacitance with voltage and frequency is due to the low defect density available at the bulk
and near to the metal-insulator interface.
The dispersion of capacitance with frequency is explained in electrode polarization
model, developed by Beaumont and Jacobs [Beaumont and Jacobs, 1967]. The model is
helpful to understand the polarization process and nature of defects. Oxygen vacancies are
considered as dominant intrinsic defects in many high-k oxides, however, the density of
defects depends on the oxide growth processes. These vacancies lead to localized
conduction by hoping of electrons. While AC signal is applied, the mobile charges form a
double layer near electrodes. The double-layers are considered as injected free electrons
from electrode or oxygen vacancies [Gonon and Vallae, 2007]. For applied bias, the mobile
charges are accumulated at a distance Ld from the electrode, called Debye length. This
41
-2
0
2
4
6
8
10
12
5.0
5.2
5.4
5.6
5.8
6.0
6.2
6.4
1K 5K 50K 200K 500K 800K
Ca
pa
cit
an
ce d
en
sity
(fF
/μm
2)
Frequency (Hz)
Measured (25deg)
Model (25deg)
Measured (125deg)
Model (125deg)
Measured σ (25deg)
Measured σ (125deg)
Co
nd
uctiv
ity σ
(×1
0-1
5 S/c
m)
Figure 3.9: Frequency dependence of capacitance of 49.3nm thick Al2O3 MIM capacitor atdifferent temperatures
modulation of space charge region under the AC field is referred as “Electrode
polarization”. According to this model, the capacitance is [Gonon and Vallae, 2007],
C =Cm
[1+
Ac
ω2nτ2n
](3.3.4)
where Cm is the capacitance for no electrode polarization, expressed as Cm = ε0εrS/L,
with top electrode area S and oxide thickness L. In Eq. 3. 3. 4, the slowly varying quadratic
second term has (ωτ)2n, called Jonscher response, with 0 < n < 1. ω and τ are angular
frequency of AC signal and relaxation time of oxide respectively. Parameters Ac and τ are
expressed as,
Ac =2
(2+ρ)2LLd
, (3.3.5)
τ = τ01
(2+ρ)
LLd
. (3.3.6)
In equations (3. 3. 5) and (3. 3. 6), the intrinsic relaxation time τ0 = ε0εr/σ and Debye
length Ld = (ε0εrkBT/Ntq2)1/2, where Nt is density of intrinsic defects and σ is
42
conductivity of dielectric. ρ is called “blocking parameter” which is a measure of the
electrode transparency. ρ = αν(L/D)exp(−Ei/kBT ) where α and ν are hopping distance
and hopping frequency normal to the interface, respectively and D is bulk diffusion
coefficient. For strongly injecting contacts, like ohmic contacts, ρ tends to infinity which
further gives Ac = 0 and C ≈ Cm. This indicates that space charge is not formed at the
metal-dielectric interface. In contrast, when the contact is not injecting any charges, Ac is
very large and ρ is very small. This describes importance of the effect of space charge
[Gonon and Vallae, 2007].
From [14], a = 0.5nm, ν = 1012Hz and interfacial energy barrier for Al/Al2O3 Ei =
0.98eV . The measured conductivity of the anodic oxide is shown in Fig. 3. 9 which yields
Ld = 1.1nm and ρ = 3.4± 2. For this model, the best fit has been obtained by considering
Nt = 3.2×1015/cm3 and n = 0.072±0.001 at 25oC. The model fits at n = 0.09±0.001 for
125oC for the same defect density. The model is extended to calculate temperature dependent
capacitance at 100 KHz and 1 MHz with the same settings and shown in Fig. 3. 9.
The model confirms that the low defect density (∼ 1015/cm3) at the bulk insulator
results stable frequency and temperature response. The second term of the model refers the
contribution of relaxation polarization during formation of capacitance. It is slowly varying
in the order of n = 0.072±0.001 as frequency increases. This slow variation of capacitance
indicates that the polarization process is dominated by ionic or displacement polarization
rather than the relaxation polarization. This agrees to the results reported by L. M. Kosjuk
et al [Kosjuk and Odynets, 1997].
3.3.4 Leakage characteristics and Conduction mechanisms
The measured leakage current density for three samples is shown in Fig. 3. 10. The sample
Al-1 shows the leakage current density of 1nA/cm2 for applied voltages up to 5V which is
much lower than ITRS recommendation for the year 2011. The breakdown voltage obtained
from the leakage characteristics is ∼12.5V for thickness of 14.3nm which agrees to the
43
-12
-11
-10
-9
-8
-7
-6
-5
-4
-3
-2
0 3 6 9 12 15 18 21 24
log
(J)
(A
/cm
2)
Applied Voltage (V)
14.3nm
28.8nm
49.5nm
1st knee
kink
2nd knee
ITRS-2018
Figure 3.10: Measured leakage current density for three samples
earlier results [Lhymn et al., 1986]. The conduction mechanism of barrier type anodic
alumina is analyzed based on Schottky emission (SE), Poole-Frenkel (PF) emission and
Trap Assisted Tunneling (TAT). Higher slope at very low voltage indicates the schottky
thermionic emission of electrons to the unoccupied defect or trap states near metal-insulator
interface. Low field current density is dominated by TAT, which depends on temperature,
defect density and trap well depth. High field is dominated by PFT which accounts the
trapped electron enhanced from defect states to conduction states of the dielectric.
Transition from Schottky emission to TAT can be observed by “1st knee” point. The knee
point varies with current density and thickness of oxide layer. Morgan et al have focused on
the changes of barrier height due to oxide thickness [Morgan et al., 1980]. The barrier
height of Al/Al2O3 interface is expressed as φ = q2NtL/2ε0εr. The barrier height of the
bottom electrode decreases as the thickness increases. This indicates that the trap wall depth
of oxygen vacancies near metal-insulator interface is deep. On the other hand, the “kink”
point can be observed during transition from TAT to PFT. The kink point from 1st knee
point at the low field is almost a straight line with similar slope at all thicknesses. This
indicates the uniform trap density and deep trap energy over oxide, which is a useful feature
44
-50
-45
-40
-35
-30
-25
-20
-15
2509 2774 3015 3239 3448 3645
ln(J
/E)(
A/c
m.V
)
E1/2(V/cm)1/2
Measured
PF-model
L=14.3nm
Slope βPF=0.0016
Figure 3.11: PFT fit with measured leakage characteristics
for tunnel barrier structure.
The high fields are dominated by PF emission, during hopping conduction of the
trapped charges between trap potential wells. This hopping rate is further increased by
applied voltage and temperature. The PF emission current density is expressed as [Ding
et al., 2004],
JPF =CEexp{− 1
ξ kBT
(qφPF −βPF
√E)}
(3.3.7)
where C is pre-exponential factor, φPF is Poole-Frenkel trap energy and the slope of
logarithmic fit is βPF =√
q3/πε0εr. Fig. 3. 11 shows the leakage characteristics with
ln(J/E) as a function of√
E, fitted with PF emission mechanism for 14.3nm thickness. The
best fit occurs at φPF = 1.47eV for the dynamic relative permittivity of Al2O3 εr = 3.25 [Ding
et al., 2004] at higher field. From Fig. 3. 10, Poole-Frenkel saturation (PFS) is observed after
the 2nd knee point. Trap barrier height is reduced to zero for voltages at or above 2nd knee
point, thus the charged (Coulombic) traps have no effect on the carriers [Southwick III et al.,
2010]. This PF saturation dominates PFT at higher thickness which ensures the barrier height
reduction of the metal-oxide interface for thicker anodic oxides. It’s clear from the tunneling
mechanisms that bulk oxide has very low and non-uniform defect profile. The Schottky
45
-12
-11
-10
-9
-8
-7
-6
-5
-4
-3
0 4 5 6 7 8 9 10 11
log
(J)
(A/c
m2)
Electric Field (MV/cm)
25oC50oC75oC100oC125oC
kink
25oC
50oC
75oC
100oC
125oC
ITRS-2018
Figure 3.12: Leakage current density for various temperatures from 25oC to 125oC
emission at the very low field indicates the higher deep trap states or oxygen vacancies near
metal insulator interface. This ensures that the bulk barrier anodic oxide is highly crystalline
whereas the surface or outer layer is amorphous. The insolubility of inner layer and slight
solubility of outer layer with electrolyte lead to such defect profile.
3.3.5 Reliability Studies
In this section, the temperature and stress dependent leakage mechanisms and related
reliability issues are studied. The time to break down and trap characteristics of anodic
Al2O3 MIM capacitor are studied using constant voltage stress (CVS) experiments in detail.
Temperature dependent leakage characteristics
The leakage current density for various temperatures from 25oC to 125oC has been measured
and shown in Fig. 3. 12. The higher slope at low fields indicates that the metal-insulator
interface is dominated by SE of electrons. As per the model proposed by Atanassova et
al [Atanassova et al., 2008], the low fields are dominated by TAT and the high fields are
dominated by PFT. The transition from TAT to PFT is observed by the kink which occurs
46
1
1.05
1.1
1.15
1.2
1.25
1.3
1.35
1.4
1.45
1.5
0 5164 7303 9309 10646
Sch
ott
ky
Ba
rrie
r (e
V)
E1/2((MV/cm)1/2)
25deg
50deg
75deg
100deg
125deg
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
0 14 22 26 29
FP
-Tra
p b
arr
ier
hei
gh
t (e
V)
E1/2 ((MV/cm)1/2)
25deg50deg75deg100deg125deg
Figure 3.13: (a) Extracted barrier height at various temperatures for A6 sample, (b) Extractedtrap barrier height for various temperatures.
at different values of field strength as temperature varies. The kink disappears after 100oC
which means PFT has become a dominant tunneling for a wide range of field. This ensures
that the traps are deep and highly sensitive to temperature. Similar observations were made
by others on ALD Al2O3 [Specht et al., 2004].
To ensure these speculations, the Schottky barrier height and trap barrier height are
extracted using SE and PF conduction models. These models were explained in Chapter-1
in detail. With βSE =(q3/4πε0εr
)1/2 and βPF =(q3/πε0εr
)1/2, the conduction models are
expressed as [Chakraborty et al., 2005],
JSE = ART 2exp{− 1
kBT
(qφB−βSE
√E)}
(3.3.8)
JPF =CEexp{− 1
ξ kBT
(qφPF −βPF
√E)}
(3.3.9)
Barrier height (φB) and trap barrier height (φPF ) are extracted and shown in Fig. 3. 13
(a) and 3. 13 (b). Here dielectric constant εr is assumed as 9. It is observed that the Schottky
barrier height is ∼ 1.25eV while extrapolating the calculated barrier height at high fields to
zero field. This high barrier height is responsible for low leakage at low fields. Extracted
trap barrier heights at high fields for various temperature are shown in Fig. 3. 13 (b). The
intrinsic trap barrier height is obtained by extrapolating the linear fit to zero field which yields
φPF = 1.47eV . This agrees with earlier results [Yeh et al., 2007]. More over, the trap height
47
-7
-6
-5
-4
-3
-2
-1
0
0.01 0.1 1 10 100 1000 10000lo
g (
J)
(A/c
m2)
Stress time (sec)
8V
9V
10V
Figure 3.14: Measured leakage current density vs. stress time at various applied voltages(CVS)
1E+00
1E+01
1E+02
1E+03
1E+04
1E+05
1E+06
1E+07
1E+08
1E+09
1E+10
0 3 7 10 13 16
TB
D (
sec)
Anode voltage (V)
14.3nm
28.8nm
49.5nm
10 year
Figure 3.15: Measured TBD for various dielectric thicknesses at room temperature
reduces for increase in temperature with a rate of 0.063eV/oC. The stable characteristics of
Schottky and trap barrier heights with temperature is due to improved lattice arrangement
during anodic polarization. This enhances the breakdown field strength of Al2O3.
Constant voltage stress (CVS)
CVS measurement has been carried out by measuring leakage current density as a function
of stress time at constant applied voltage. This stress measurement is useful to predict
Time-to-Breakdown (TBD) and life time of semiconducting structure at real time
conditions. The TBD is one of the important parameters to assess the reliability of
capacitors which ensures the life time of the device. Measured CVS results of Al2O3 MIM
capacitor are plotted in Fig. 3. 14. Leakage current is decreasing for the stress time up to
10sec for applied voltage of 8V. This behavior describes the saturation of the electron
48
10
100
1000
10000
100000
1000000
10000000
100000000
1E+09
1E+10
0 2 4 6 8 10
TB
D (
sec)
Anode voltage (V)
25oC
125oC
10 year
25oC
50oC
Figure 3.16: Measured TBD of 49.3nm thick MIM capacitor at various temperatures
trapping or newly created traps due to electrical stress. After 10sec, leakage current
increases due to generation of positive defects which causes dielectric breakdown [Remmel
et al., 2003]. TBD is measured at various constant voltage stress for different thicknesses of
anodic alumina at room temperature and plotted in Fig. 3. 15. By extrapolating the
measured values of TBD to 10 years line, it shows that anodic Al2O3 MIM capacitors can
operate upto 10 years for the continuous stress of 2V.
The TBD is measured at various temperatures for the applied voltages between 4V to
10V and shown in Fig. 3. 16. By similar extrapolation of extracted TBD data, TBD will
reach to 10 years for the operating voltage of 2V at room temperature. Similarily the TBD at
125oC, the TBD of 10 years can be achieved for applied voltage stress of 1.7V. This shows
that the barrier type anodic oxides are highly reliable at higher temperatures. This is due to
the strong ionic bond and high breakdown field of Al2O3.
Trap characteristics
The trap density is measured using constant stress time bias. This method is also called as
ramp voltage stress (RVS). The applied stress voltage is increased as ramp signal for every
constant period of time, say 5mS. Fig. 3. 17(a) and 3. 17(b) are showing the leakage
characteristics for the constant stress times from 500µs to 5s under positive and negative
bias conditions respectively. The trap density is calculated using double I-V method [Liu
49
et al., 1991],
800E-9
850E-9
900E-9
950E-9
1E-6
16.00 16.15 16.30
Cu
rren
t d
en
sity
(A
/cm
2)
Voltage (V)
Fresh
5s
500ms
5ms
500μs
600E-9
650E-9
700E-9
750E-9
800E-9
850E-9
900E-9
950E-9
1E-6
-15.00-15.06-15.12-15.18
Cu
rren
t d
en
sity
(A
/cm
2)
Voltage (V)
Fresh
5s
500μs
5ms
500ms
(a) (b)
Figure 3.17: J-V characteristics for the constant stress times from 500µs to 5s under (a)positive and (b) negative bias
6.1
6.2
6.3
6.4
6.5
6.6
6.7
6.8
500μs 5ms 500ms 5s
Tra
p d
ensi
ty (
10
5/c
m2)
Stress Time
14.3nm
28.8nm
49.5nm
Figure 3.18: Calculated Trap density for various stress times in three samples
Nt =εox
qTox(∆V−−∆V+)
where ∆V+ and ∆V− are the voltage shifts on the applied stress under positive and
negative biases, respectively. Fig. 3. 18 shows the trap density calculated for different stress
times. Trap density increases about ∼5% from stress time 500µs to 5s, the increment is due
to the newly created traps. The difference between trapped electron density and newly
produced trap density is negligible, which in turn reduces sensitivity of capacitance to
frequency variation. On the other hand, the trap density calculated for other samples exhibit
maximum difference of 0.1× 105/cm2 though the thickness increases. This indicates the
50
Al2O3 Thermal ALD Porous Barrier typeDeposition oxidation anodization Anodization
[Chen et al., 2002] [Ding et al., 2007] (Hourdakis, 2010) [current work]
Capacitance density5 6.05 5.1 6.01
( f F/µm2 )Leakage current density
10-8 - 10-9 10-11at 1V (A/cm2)
Leakage current density10-7 10-8 10-9 10-10
at 2V (A/cm2)Breakdown filed
8.61 3.6 8.77(MV/cm)
VCC>1000 795 >1000 400
(ppm/V )TCC
200 - - 150(ppm/oC)
Variation of10% - 40% 6%
capacitance (%)
Table 3.1: Performance of various processing techniques to deposit Al2O3 for the thicknessof ∼15nm
oxide region should have very less defects where the majority of traps are located nearby
top/bottom electrode.
3.4 Summary
Table-1 specifies the performance of Al2O3 MIM capacitors using various dielectric
deposition techniques. Thermal oxidation of Al shows low capacitance density and high
leakage current due to high oxygen vacancies and incomplete oxidation of Al even at 400oC
[Chen et al., 2002]. Also the fabricated porous anodic oxide MIM capacitor results in
reduced breakdown field and high sensitivity to the frequency. This indicates the sign of
high defect/trap density at the interface and bulk because of solubility in the electrolyte
during anodization [Hourdakis and Nassiopoulou, 2010]. ALD and current work on barrier
type anodic MIM capacitors exhibit excellent reliability and high capacitance density.
However, the anodization provides best quality oxides at lowest fabrication cost.
This chapter presents fabrication and characterization of MIM capacitors with barrier
type anodic Al2O3. Anodic alumina MIM capacitor shows high capacitance density
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(6 f F/µm2) and low leakage current density of < 1nA/cm2 for the applied voltages up to
5V which are meeting the requirements of ITRS for the year 2015. The capacitance exhibits
low dependency with frequency as compared to the earlier results. The defect free oxidation
of Al2O3 shows improved polarization which helps to reduce capacitance dependence on
frequency. These results suggest that the anodization is a high quality oxidation technique
of high-k metal oxides for MIM capacitors and other device applications at low fabrication
cost.
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