Dielectric breakdown and failure of anodic aluminum oxide films forelectrowetting systemsM. Mibus, C. Jensen, X. Hu, C. Knospe, M. L. Reed et al. Citation: J. Appl. Phys. 114, 014901 (2013); doi: 10.1063/1.4812395 View online: http://dx.doi.org/10.1063/1.4812395 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v114/i1 Published by the AIP Publishing LLC. Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors

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Dielectric breakdown and failure of anodic aluminum oxide films forelectrowetting systems

M. Mibus,1 C. Jensen,2 X. Hu,3 C. Knospe,4 M. L. Reed,3 and G. Zangari1,a)

1Department of Materials Science and Engineering, University of Virginia, Charlottesville, Virginia 22904, USA2Department of Chemical Engineering, University of Virginia, Charlottesville, Virginia 22904, USA3Department of Electrical and Computer Engineering, University of Virginia, Charlottesville, Virginia 22904, USA4Department of Mechanical and Aerospace Engineering, University of Virginia, Charlottesville,Virginia 22904, USA

(Received 3 April 2013; accepted 13 May 2013; published online 2 July 2013)

We study electrical properties and breakdown phenomena in metal/aluminum oxide/metal and

electrolyte/aluminum oxide/metal contacts, with the aim to achieve a better understanding of

failure modes and improve the performance of model electrowetting systems. Electrical conduction

in anodic aluminum oxide dielectrics is dominated by the presence of electrically active trapping

sites, resulting in various conduction mechanisms being dominant within distinct voltage ranges

until hard breakdown occurs. Breakdown voltage depends on its polarity, due to the formation of a

p-i-n junction within the oxide; such asymmetric behavior tends to disappear at larger oxide

thickness. Electrolyte/dielectric contacts present an even more pronounced asymmetry in

breakdown characteristics: a cathodic bias results in breakdown at low voltage, while under anodic

bias high field ionic conduction starts before breakdown occurs. These phenomena are interpreted

in terms of electrochemical reactions occurring at the surface: cathodic processes contribute to

oxide dissolution and failure, while anodic processes result in additional oxide growth before

breakdown. VC 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4812395]

INTRODUCTION

Electrowetting on dielectric, or EWOD, refers to the

change in apparent contact angle when a voltage is applied

between a liquid drop and a surface. The use of a dielectric

in place of a conductive surface allows the application of

large voltages while avoiding electrolysis of the fluid.1 An

EWOD device can therefore operate over a large voltage

range allowing for significant contact angle change without

the onset of spurious effects. EWOD has been successfully

exploited in various microfluidic devices including electro-

des to move and mix fluids,2,3 electrically controllable mi-

crolenses,4 light transmission through optical fibers,5

reflective displays for electronic paper,6 and capillary force

actuators.7 To first order, the voltage dependence of the con-

tact angle is quantified by the Young-Lippmann equation

cosðhÞ ¼ cosðh0Þ þ1

cLG

1

2

ere0

tV2; (1)

where h is the apparent contact angle of the liquid with

applied voltage, h0 is the equilibrium contact angle, clg is the

surface tension at the gas-liquid interface, er is the dielectric

constant of the dielectric layer, e0 is the permittivity of free

space, t its thickness, and V is the applied voltage. The per-

formance of current EWOD systems is limited by the use of

relatively thick (�10�6 m) dielectric films, which according

to Eq. (1) require a high voltage to achieve appreciable

changes in contact angle.8 The most direct means to mini-

mize the voltage applied during operation is to increase the

dielectric layer capacitance by using a thinner layer and/or

materials with high dielectric constant. One concern with the

former approach is that by reducing dielectric thickness, the

electric field across this layer would increase, magnifying its

susceptibility to breakdown and failure. The use of materials

with high dielectric constant, on the other hand, is a promis-

ing avenue to enhance device performance. Another limita-

tion of current EWOD systems is contact angle saturation,

which is observed at high voltages and has been also linked

to the onset of breakdown phenomena;9,10 delaying or alto-

gether avoiding breakdown would therefore result in larger

contact angle variation, enhanced performance, and

increased reliability.

In order to achieve large values of h0 and large changes

in h, actual EWOD systems require a hydrophobic coating in

direct contact with the liquid; when using an oxide film as

dielectric the wetting angle at equilibrium would be low, and

a fluoropolymer layer must thus be added to the film stack.11

This work is mostly focused on the dielectric properties of

the oxide, and in order to simplify the system under study we

avoid using a hydrophobic topcoat; we show however that

addition of such topcoat has the effect of further delaying the

onset of breakdown phenomena.

Several valve metal oxides exhibit high values of the

bulk dielectric constant: Al2O3(er¼ 9), Ta2O5(er¼ 22), ZrO2,

and HfO2(er¼ 25).12 These oxides can be grown by film dep-

osition methods, using sputtering or evaporation, or can be

obtained by UV-assisted or electrochemical oxidation (anod-

ization) of metallic film precursors. Anodization entails

application of a positive voltage to a metal film immersed

in an appropriate electrolyte, converting the metal to the

corresponding metal oxide. This method presents several

a)Author to whom correspondence should be addressed. Electronic mail:

gz3e@virginia.edu

0021-8979/2013/114(1)/014901/8/$30.00 VC 2013 AIP Publishing LLC114, 014901-1

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advantages: anodization can be performed with simple

instrumentation at low temperatures, and, in addition, setting

the applied voltage gives precise control over the final thick-

ness of the oxide layer.

Electrochemical anodization of all valve metals has been

demonstrated;13 however, anodization of aluminum is by far

the best understood and most easily implemented. The nature

of the anodic film formed on aluminum depends mainly on

the anodization electrolyte: a thin and compact barrier layer14

is formed in neutral electrolytes where the oxide film is insol-

uble, while a porous structure15 is obtained in acidic electro-

lytes. In EWOD applications, a barrier layer is preferred since

its smoothness limits contact angle hysteresis. During the

anodization process, the oxide film grows via field-induced

migration of the metal ion and the oxygen anion in opposite

directions, resulting in growth occurring simultaneously at the

metal/oxide and oxide/electrolyte interface, where the two

species react. The final thickness of the oxide layer is propor-

tional to the anodization voltage, with a slope (anodization ra-

tio, AR) of �1.1 to 1.4 nm/V, depending on the anodization

solution.14 The resulting oxide is amorphous and therefore

presents a high density of point defects, such as vacancies,

interstitials and impurities from the electrolyte, all of which

may generate electrically active trapping sites within the ox-

ide. These defects are responsible for the (low) electronic con-

ductivity of dielectric oxide films.

Degradation and failure in EWOD systems either may

result from phenomena occurring at the liquid/dielectric

interface or may originate within the solid oxide, from the

accumulation of point defects generated by electron transport

through the dielectric; breakdown, consisting in a sudden

increase in conductivity and loss of functionality, is observed

when a critical density of defects is reached, sufficient to

form a percolative and conductive path between the electro-

des.16 Breakdown has been studied extensively in the solid

state on silicon oxide, in the context of gate oxide degrada-

tion in CMOS,17 and more recently in Al,18 and high-edielectric films.19 In contrast, the current understanding of

degradation and failure at oxide/electrolyte interfaces is lim-

ited. An aluminum oxide/electrolyte interface is known to

exhibit rectification, with a cathodic polarization leading to

large currents.20 The phenomenon was ascribed to hydrogen

reduction occurring at the oxide surface, leading to proton

conduction within the oxide via defects in the film.21 This

reduction current may be accompanied by an additional ca-

thodic phenomenon, i.e., reduction of the oxide, leading

eventually to dissolution of the oxide.22 Stability of the oxide

film is further affected by the nature of the electrolyte anion,

with Cl� being especially harmful.23 More recently, Raj

et al.24 have demonstrated the importance of electrolyte

chemistry in enhancing the performance of CYTOP (Asahi

Glass), an amorphous fluoropolymer coating serving as a

hydrophobic topcoat, showing that the use of large size cati-

ons and anions hinders dielectric failure. The deleterious

effect of polarity and Cl� has been further confirmed in

Ta2O5 EWOD devices, where a positive bias resulted in a

quicker and irreversible decrease in contact angle.25

In this report we investigate breakdown and dielectric

failure of anodized aluminum oxide barrier films with

thicknesses ranging from 20 to 60 nm. To this end, we use

both an all solid state metal/oxide/metal (MOM) contact and

an electrolyte/oxide/metal (EOM) contact to compare and

contrast failure due to conduction within the oxide with that

due to electrochemical processes at the interface. We find that

MOM contacts exhibit non-linear, asymmetric characteristics,

due to an asymmetric charge distribution in the oxide; sharp

breakdown is observed at potentials which increase with oxide

thickness. EOM contacts present a more pronounced asymme-

try in their I-V characteristics and fail in correspondence of

the onset of charge transfer reactions at the surface.

EXPERIMENTAL SECTION

Materials

Aluminum films of 100 nm thickness were grown by

e-beam evaporation (CHA Industries, Fremont, CA) onto the

native surface of a Si wafer, pre-coated with a 5 nm thick

titanium adhesion layer. Aluminum anodization was per-

formed at room temperature in a two electrode electrochemi-

cal cell with the electrodes in a vertical configuration, using

a platinum mesh counter electrode. The anodization solution

was 30 wt. % ammonium pentaborate (NH4B5O8) in 99.99%

ethylene glycol. The anodization voltage, V, varied from

10 V to 40 V. The thickness of the resulting barrier film

depends upon V and is calculated by the anodizing ratio

(nm/V).

MOM contacts were obtained by depositing a 100 nm

thick Aluminum film with e-beam evaporation on top of the

Al oxide barrier layer. Eight independent Al contacts with

0.80 mm2 area were grown at distinct locations on the oxide

using a shadow mask. EOM contacts were prepared by care-

fully positioning a 1.5 ll droplet of a.05 M sodium sulfate so-

lution on the oxide surface with a pipette.

All solutions described above were prepared from Milli-

Q water (resistivity 18.2 MX cm), using chemicals from

Sigma-Aldrich with >99% purity.

Film characterization

The oxide thickness was measured with a spectroscopic

ellipsometer (HORIBA Jobin Yvon, Irvine, CA); the mea-

surement was performed at several points across the film

area, showing less than 1% variation. Thickness was further

confirmed by monitoring the charge density Q passed during

the anodization process and using t¼QM/zFq, where M is

the atomic weight of the oxide, z the number of transferred

electrons in an elementary reaction, F Faradays’ constant,

and q the oxide density. Surface morphology of the films

was investigated with a field emission scanning electron

microscope (JEOL 6700F).

The current-voltage characteristics were recorded in

both the MOM and EOM configurations using two probes

controlled through a HP 4145b semiconductor property ana-

lyzer. Data were collected at 0.1 V intervals while sweeping

the voltage at a rate of 0.1 V/s. Probe positioning was con-

trolled through a Micromanipulator 7000 probe station with

gold plated probe needles. In the following, a bias will be

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designated as positive if a positive voltage is applied to the

Al contact (MOM) or to the electrolyte (EOM).

RESULTS AND DISCUSSION

Oxide growth

Figure 1 shows the aluminum oxide thickness, as deter-

mined by ellipsometry, vs. anodization voltage. As expected

the relationship is approximately linear, with an anodizing ratio

of �1.2 nm/V. The sample anodized at 10 V shows a positive

deviation from the extrapolated linear trend; this behavior is

ascribed to a change in the ionic migration process, dominated

at low voltage by ion injection in the oxide, and at higher volt-

age by bulk ion migration, leading to an increase in average

growth rate at larger thickness.14 On the same grounds, the

extrapolation at low voltages should not be taken literally, since

under these conditions both ion migration and diffusion may

contribute to ionic transport. The oxide films are very smooth

and continuous, as determined by SEM imaging.

Metal-oxide-metal contacts

Current-voltage I-V characteristics for MOM contacts

were recorded for various oxide film thickness and typical

results are reported in Fig. 2(a). The characteristics are

highly non-linear and are asymmetrical with respect to polar-

ity, with the positive polarity (positive voltage applied to the

top Al contact) always showing breakdown at a lower volt-

age. A sharp, hard breakdown, leading to a steep current

increase up to the maximum current allowed by the available

instrumentation, is observed in all cases. The electrical

behavior of the oxide is reversible if the applied voltage is

maintained below the breakdown voltage VB, but the

changes in conductivity are irreversible if the region of

breakdown is reached. Fig. 2(b) reports VB and the corre-

sponding breakdown field EB¼VB/t, measured under both

polarities as a function of the oxide film thickness. For each

sample, the error bars identify the highest and lowest value

of VB, EB measured on that sample, and the symbols repre-

sent the average value. It is noted that both the scatter in VB,

EB and the difference between VBþ(EBþ) and VB�(EB�)

decrease with increasing thickness (i.e., increasing anodiza-

tion voltage).The breakdown field for the oxides ranges

between 3-7 MV/cm with the value leveling to 7 MV/cm in

both polarities with increasing film thickness (Fig. 2(b)),

similar to the values observed in other anodic aluminum

oxide films,26 but higher than thermally oxidized

aluminum(5 MV/cm).18

The observed asymmetry in the I–V characteristics can

be traced back to an asymmetry in the Al oxide film struc-

ture. During anodization, the large electric field in the oxide

causes injection of metal ions from the metal side, and of

anions from the electrolyte (O2�, OH�, borate), into the ox-

ide. Opposite gradients in the concentration of both cations

and anions are therefore present within the oxide, leading to

the formation of a p-intrinsic-n (p-i-n) junction, where the p

and n region are about 5 nm thick and their doping polarity is

determined by an excess of anions and cations, respec-

tively.27 The presence of a p-i-n junction naturally leads to

asymmetric characteristics; in addition, the degree of asym-

metry would decrease with increasing oxide thickness since

under these conditions the intrinsic region makes up most of

the oxide thickness and would dominate the electrical behav-

ior. Under positive bias the p-i-n junction is forward biased,

and the resulting current is larger. A larger current flowing

through the oxide would accelerate the breakdown process,

leading to a lower breakdown voltage, as observed. It should

be noticed also that during anodization the sample is sub-

jected to negative bias polarization, according to the defini-

tion adopted here; the fact that a higher breakdown voltage

is observed under these conditions suggests therefore that

transport of charges becomes more difficult when the oxide

is subject to the same bias present during anodization.

Electronic conduction processes in MOM contacts

The highly defected structure of anodized Al oxide gives

rise to localized excess or deficit of oxygen, Al, or impurities,

yielding a high density of trapped charges which act as electri-

cally active defects.28–30 The defect concentration has been

quantified for a wide variety of passive metal films, yielding

values in the range of 1018-1020 cm�3.31 Specific values for

aluminum oxide barrier layers have recently been measured at

1018-1019 cm�3, depending on thickness.32 Significant elec-

tronic conductivity may therefore be present, and various elec-

tronic conduction processes become thus possible.33 These

include Schottky emission (SE), Poole-Frenkel emission (PF),

and Fowler-Nordheim tunneling (FN);34 the total electronic

current would therefore be j¼ jSEþ jPFþ jFN.

Schottky emission consists in the injection of thermally

activated carriers from a metal contact into the oxide through

a potential barrier u lowered by an applied electric field.

Deformation of this barrier due to image charges is limited

in high-e materials; therefore, this effect should not be as im-

portant as in SiO2. However, due to the highly defective na-

ture of the metal oxide layer interface, the barrier could be

reduced even for weak image charge effects. jSE is described

by the Schottky-Richardson equation for thermionic

emission35

FIG. 1. Thickness of anodized aluminum oxide films vs. anodization volt-

age. Thickness measurements by ellipsometry are in close agreement with

calculations from the charge density.

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lnðJÞ ¼ b2kT

ffiffiffiEpþ lnðAT2Þ � q/

kT; (2)

b ¼ q3

4pere0

; (3)

where b quantifies the barrier reduction, E is the electric

field, T the temperature, A is a constant, and the other sym-

bols have the usual meaning. Fitting ln jSE vs. sqrt(E) allows

determination of er. The resulting value is 10.7 with minimal

variance across thicknesses, comparable to the reported

range of 7-12.8 for anodic Al2O3.13

Poole-Frenkel emission describes charge injection

between localized charged trap sites, again triggered by a

field-induced reduction of the energy barrier, which leads to

hopping processes. P-F emission is particularly relevant for

oxide thickness above 25 nm, where the relative importance

of Schottky emission would decrease. The Poole-Frenkel

current is described by Eq. (4)

lnJ

E

� �¼ b

2kT

ffiffiffiEpþ lnðCÞ; (4)

where C represents a proportionality constant for the conduc-

tivity in the absence of electric field. Fitting a linear relation-

ship to ln(j/E) vs. sqrt(E) provides again an estimate of the

dielectric constant, yielding a value between 10 and 15, inde-

pendent of thickness. This current saturates when the applied

field is sufficient to remove all charges from their traps.

Under these conditions the bending of the energy barrier is

very large, increasing the probability of tunneling.

Fowler-Nordheim tunneling defines tunneling through a

barrier reduced by an applied electric field. The equation

describing this phenomenon is

FIG. 2. (a) I–V characteristics for alu-

minum oxide films of various thickness

in a metal-oxide-metal configuration

operating under both polarities. (b)

Breakdown voltage VB and breakdown

field EB vs. film thickness for positive

(black, square) or negative (red, trian-

gle) polarity applied to the top contact.

The error bars represent max and min

values for the data scatter with the av-

erage value as the data point.

014901-4 Mibus et al. J. Appl. Phys. 114, 014901 (2013)

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lnJ

E2

� �¼ lnðAÞ � B

E; (5)

where

A ¼ mq3

m�8ph/;

B ¼ 8p3

2m�

h2

� �1=2/3=2

q:

A and B are constants dependent on the metal-insulator bar-

rier u and the electron effective mass in the oxide m*. FN

tunneling describes the high field conduction region just

before the onset of breakdown.

It is possible to fit the experimental I-V characteristics

assuming that the three processes described above dominate

within distinct applied potential regions. The transition from

SE to PF emission in 56 nm oxides is marked by a change in

the slope in the semi-logarithmic characteristics (Figure

3(a)); a narrow voltage region just before the irreversible

increase in current on the other hand can be fit with a FN tun-

neling characteristics. 32 nm thick layers in contrast (Figure

3(b)) are better fit with a superposition of SE and FN tunnel-

ing only; this is consistent with thinner films being less de-

pendent on charge traps for conduction.

In summary, the current flowing through Al oxide

dielectric films is electronic in nature and is described by a

combination of emission, hopping, and tunneling processes

(Figure 3(c)) that become dominant within distinct applied

voltage ranges. The order in which they are observed

depends on the energy barrier for the corresponding process,

with tunneling being active only at large voltages.

Accumulation of the damage originated by the applied field

leads to the formation of a percolative path, increase in con-

ductivity, and irreversible damage, eventually leading to

dielectric failure. The observed asymmetry in the character-

istics is explained by the formation of an asymmetric charge

distribution within the oxide as a consequence of the anod-

ization process.

Conduction in electrolyte-oxide-metal contacts

Current-voltage characteristics for EOM contacts were

also recorded for oxide films of different thickness; the cor-

responding results are shown in Fig. 4. It is noted that the

asymmetry of the characteristics in this case is even more

pronounced than with the MOM contacts. Let us examine

the negative and positive bias regions separately.

The I-V curve under negative bias reproduces in the low

voltage range the trend observed with the MOM contacts;

the current density at corresponding voltage however is

about two orders of magnitude lower, probably due to the

sluggishness of the charge transfer reactions necessary to

generate ionic carriers in the electrolyte. Since the current in

this region is very small, no significant potential drop is

expected within the electrolyte, due to its high conductivity

relative to the dielectric. This is consistent with the fact that

this section of the characteristics can be fit through a combi-

nation of Schottky and PF emission, as shown in Figure 5(a).

What are the charge carriers in this potential range? We

hypothesize that spurious oxidation reactions occur at the ox-

ide/electrolyte interface, generating electrons that migrate

through the oxide, reaching the base metal. Afterwards, the

FIG. 3. (a) Fit of the experimental I-V characteristics of a 56 nm thick Al oxide

film to a combination of dielectric leakage mechanisms; each process in turn is

assumed to dominate conduction in the voltage region where the corresponding

characteristics applies: Schottky emission (red), Poole-Frenkel (blue), Fowler-

Nordheim (orange). (b) I–V characteristics of a 32 nm thick oxide layer; a good

fit could be obtained by assuming only SE and FN conduction mechanisms. (c)

Schematic for the MOM electronic conduction mechanisms.

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slope of the I-V curve increases and the sharp breakdown

process observed in MOM contacts is replaced by a gradual

current increase which eventually approaches saturation. The

potential at which the slope increases corresponds closely to

the anodization voltage. Taking into account that a negative

bias for the EOM contact corresponds to an anodic bias for

the oxide, we hypothesize that the observed current in this

region is an ionic current I, which is described by an expo-

nential dependence of current on applied field, E,

I¼ a*exp(bE), where a and b are temperature dependent con-

stants;36 the anodization potential therefore roughly sepa-

rates the potential regions where electronic and ionic

conduction dominate, respectively (Figure 5(b)). By trigger-

ing ion transport, the high voltage should result in further ox-

ide growth by anodization (Fig. 5(c)). This is the process

proposed by Dhindsa et al.37 to accomplish self-healing of

dielectrics during electrowetting operation. The actual

growth of the oxide was confirmed by applying a negative

bias to a 56 nm thick oxide using an EOM contact, sweeping

the voltage up to the saturation region for the ionic current.

After rinsing the sample, its thickness was measured by

ellipsometry and found to be 57-59 nm, 5%-8% thicker than

before. The oxide thickness increased further when holding

the current in the flat I-V region. At even higher potentials

(above 40 V in Figure 5(b)), an additional increase in current

was observed, with concurrent gas evolution; this process

corresponds to oxygen formation by water oxidation.

The oxide growth and the onset of the breakdown pro-

cess at negative bias can be monitored by sweeping the volt-

age on the same sample over several cycles while varying

the maximum applied value. Fig. 6 shows the I-V character-

istics of an EOM contact with a 38 nm thick oxide, up to the

point where ionic current flows and the current saturates

(Run 1). The second sweep is run up to the potential region

where oxygen is evolved; in this case, the increase in current

slope indicating the transition from electronic to ionic con-

duction occurs at a higher potential, suggesting that a thicker

oxide has been formed. The third voltage sweep (Run 3),

recorded after oxygen evolution on the electrode has

occurred, shows a completely different behavior, with a large

current and gas evolution occurring at much lower potentials,

below 5 V. The onset of gas evolution cannot be determined

precisely; therefore, it is difficult to correlate gas evolution

and oxide breakdown; qualitatively, however, it is possible

to associate visible gas evolution with a relatively large cur-

rent, which would trigger anodic breakdown.

The choice of electrolyte affects I-V characteristics

under negative bias. Commonly, chloride based salt solutions

are used as the electrolyte in electrowetting testing;38 this

FIG. 4. The characteristic I–V curves for Al2O3 layers of different thickness.

Polarity refers to the voltage being applied to the .05M Na2SO4 electrolyte

drop, with the ground being the base aluminum layer.

FIG. 5. (a) Typical I–V characteristics from a negatively biased EOM con-

tact consisting of a 56 nm of oxide and a .05 M Na2SO4 droplet as the top

contact. The dielectric current leakage models are fitted to the data using

Eqs. (2) and (4). (b) Two regions of the negative polarity curve are identified

where the electronic and ionic current dominate, respectively; the transition

corresponds roughly to at the anodization voltage (c) schematic of the oxide/

electrolyte interface for the negatively biased system.

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choice however is far from ideal. The harmful effect of

aggressive Cl� ions can in fact be seen in Figure 7, demon-

strating the earlier failure and increased variability of the

breakdown event. The influence of halide ions in pitting cor-

rosion is well established for passive films on aluminum.39

The proposed mechanisms include chloride ion migration

through oxygen vacancies40 or local oxide thinning due to

complex formation.41 The clear detrimental impact of chlor-

ides suggests that other solutions would provide better sys-

tem performance. In our investigation, sodium sulfate

provided a simple, reliable electrolyte that does not trigger

localized corrosion processes.

Under positive bias, the I-V characteristics exhibits an

early and sharp departure from the characteristics at negative

bias, suggesting that early breakdown is occurring already at

3–7 V. Failure seems to occur concomitantly with gas

(hydrogen) evolution, i.e., when charge transfer reactions

occur at the dielectric surface at a significant rate. Charge

transfer may occur via two mechanisms: electron transport

from the metal to the electrolyte, or cation or proton trans-

port in the opposite direction. At very low bias the I-V curve

follows that measured in the MOM contact, suggesting that

the electronic leakage current may be triggering the cathodic

reaction (water splitting and hydrogen evolution, or oxide

reduction and dissolution), generating thus protons which

may successively penetrate within and/or dissolve the oxide.

Raj et al.24 have shown that the size of the ions in the

electrowetting electrolyte may affect the breakdown process,

both under negative and positive bias. Following this lead we

have substituted Naþ (radius 0.13 nm) with Csþ (0.19 nm) or

dodecyltrimethylammonium DTAþ (3 nm––total chain

length). Figure 8 shows that substitution of Naþ with Csþ

results in a decrease of the leakage current by about two

orders of magnitude in the saturation region, and a delay in

the onset of gas evolution, which shifts from �5 V to �30 V.

Further enhancement is obtained by using DTAþ, which in

some cases does not show any gas evolution up to 40 V. The

shift in breakdown potential has been interpreted in terms of

ionic size and their diminished ability to penetrate the oxide

through defects or pinholes.24 The fact that breakdown is asso-

ciated with a sizable cathodic current, and gas evolution

FIG. 6. Three consecutive I–V characteristics recorded at a 38 nm thick ox-

ide surface using the same 0.05M Na2SO4 droplet for all curves. The curves

were run (black to red to blue) immediately following one another without

altering the configuration. Oxide thickening is observed between 1 and 2,

breakdown between 2 and 3.

FIG. 7. The influence of electrolyte anion on the oxide failure for EOM con-

tacts tested under negative polarity on 50 nm oxide.

FIG. 8. (a) Effect of cation substitution (Csþ or DTAþ instead of Naþ) in

the electrolyte on the breakdown of a 44 nm thick aluminum oxide layer in a

positively biased EOM contact. (b) Schematic of the oxide/electrolyte inter-

face under positive bias, showing the various conduction and electrochemi-

cal processes; the right panel depicts how the selective adsorption of DTAþ

may hinder gas evolution.

014901-7 Mibus et al. J. Appl. Phys. 114, 014901 (2013)

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however suggests that the phenomenon may be more com-

plex: upon gas evolution protons are evolved

(H2Oþ e�)OH�þHad, where Had indicates atomic hydro-

gen adsorbed at the dielectric surface) and penetration of

atomic hydrogen through the oxide would occur at a much

faster rate than any other ion. We hypothesize instead that

large cations may preferentially adsorb at the surface under

cathodic polarization, delaying hydrogen formation, either by

affecting the double layer structure or––in the case of

DTAþ––by physically hindering water access (Figure 8(b)).

Preliminary data collected in a three electrode set-up show in

fact that hydrogen evolution from Cs2SO4 solution occurs at

an electrode potential more negative than from a Na2SO4 so-

lution of the same concentration.

In summary, the pronounced asymmetry in the I-V char-

acteristics of EOM contacts is due to a combination of the

asymmetry in charge distribution within the oxide and the

asymmetry of electrochemical processes occurring at the

dielectric surface. An anodic potential applied to the oxide

only increases its thickness through a supplementary anod-

ization process, which occurs at potentials larger than the

previous anodization voltage. On the contrary, a cathodic

bias of the dielectric surface tends to reduce and dissolve the

oxide; this can occur as soon as current flows through the ox-

ide, the latter process being made possible by the presence of

trapped charges.

The above findings suggest that in order to enhance the

electrowetting performance of dielectric/electrolyte systems,

research should be focused on (i) decreasing leakage current

by decreasing trapped charge density: A lower leakage cur-

rent will delay the onset of breakdown in a MOM contacts as

well as the onset of electrochemical processes in EOM con-

tacts, delaying breakdown; (ii) widening the potential win-

dow of stability of the electrolyte: if cathodic and anodic

electrolyte dissociation occur at more negative and more

positive potential, respectively, the onset of breakdown will

be delayed; (iii) identifying dielectric oxides that are thermo-

dynamically stable in the electrolyte of choice under electro-

wetting conditions.

CONCLUSION

This study has investigated the underlying causes of fail-

ure in thin anodic oxide films as model EWOD systems. The

leakage current in a solid state MOM contact is described by

a combination of Schottky emission, Poole-Frenkel emission

and Fowler-Nordheim tunneling, each process dominating

within distinct voltage ranges. I-V characteristics are asym-

metric and the breakdown voltage under opposite bias is dif-

ferent. This is a consequence of the anodic oxide growth

mechanism, which results in an inhomogeneous charge dis-

tribution within the oxide, generating in turn a p-i-n junction

with asymmetric conduction properties. An electrolyte/ox-

ide/metal contact shows an even more pronounced asymme-

try in the breakdown behavior under opposite voltage bias. A

cathodic bias results in hydrogen adsorption and evolution,

leading to oxide reduction and dissolution at low applied

voltage; the preferential adsorption of large cations may

decrease the density of available sites for hydrogen

adsorption or otherwise polarize hydrogen evolution, delay-

ing degradation. When the EOM system is polarized anodi-

cally, oxide growth resumes once the anodization voltage is

exceeded, and breakdown does not occur until oxygen evolu-

tion is observed under increasing field.

ACKNOWLEDGMENTS

This work was supported by NSF grant (No. CMMI-

1030868).

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