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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