Proceedings of the ESA Annual Meeting 2005

Proceedings of the ESA Annual Meeting 2005

Editors

Joseph M. Crowley Angela Antoniu John A. Pelesko June 23–25, 2005

University of Alberta Edmonton, Canada

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Laplacian Press Morgan Hill, California

Acknowledgment

This conference was held in cooperation with the IEEE IAS-PES Northern Canada Chapter.

The papers appearing in this book comprise the proceedings of the meeting mentioned on the cover and title page. They reflect the authors’ opinions and are published as presented and without change in the interests of timely dissemination. Their inclusion in this publication does not necessarily constitute endorsement by the editors or by the ESA.

Copyright ©2005. Electrostatics Society of America. All rights reserved.

Publisher’s Cataloging in Publication Data Proceedings of the ESA Annual Meeting 2005 Includes author index. 1. Electrostatics I. Title II. Crowley, Joseph M. 1940 - II. Antoniu, Angela III. Pelesko, John A QC571 537’.2ISBN 1-885540-17-5

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Proceedings ESA Annual Meeting 2005

Voltage reduction in electrowetting-on-dielectric Stein Kuiper and Benno Hendriks Philips Research Prof. Holstlaan 4 (WAG-02) 5656 AA Eindhoven Phone: +31 40 27 46248 E-mail: [email protected] Abstract In this article several options are discussed to lower the driving voltage in electrowetting-on-dielectric (EWOD). Throughout the discussion, a liquid lens is taken as an example. The investigated variables are the thickness, dielectric constant and dielectric strength of the insulating layer, and the interfacial tension between the two liquids. Mainly by theoretical considerations, it is shown how the variables influence the driving voltage. Some results are surprising and in contrast to what is generally believed.

I. I NTRODUCTION Electrowetting is electrostatic control of the contact angle between liquids and solids. A voltage difference applied between a conducting liquid and a conducting substrate reduces the interfacial energy, which increases the degree of wetting. The phenomenon can be applied to move and shape volumes of liquids. Especially for small systems, where capillary forces dominate over gravity, electrowetting is a very attractive method to manipulate liquids. It can be used for a wide range of applications, such as for instance transport of liquids [1], variable-focus lenses [2, 3] and displays with liquid pixels [4]. The application of electrowetting for real devices has become practically feasible after the introduction of an insulating layer between liquid and substrate [5], usually called electrowetting-on-dielectric (EWOD). The insulator prevents electrolysis of the liquid, but a disadvantage is the much higher voltage that is required. Typically, voltages well above 100V are used. For battery-operated devices, this means that a high-voltage converter must be used. © 2005 Electrostatics Society of America

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S. Kuiper, B. Hendriks

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In this article, we discuss the options for reducing the driving voltage for EWOD. We try to find correlations by making simple assumptions and calculations. In order to make the considerations more concrete, we use a variablefocus liquid lens as an example throughout the article. II. LIQUID LENS In most EWOD devices a second, insulating liquid is used besides the conducting liquid. The most important functions of this liquid are lubrication, prevention of evaporation and elimination of gravity effects. In liquid lenses, it also has an optical function. Figure 1 shows the principle of EWOD in the example of a liquid lens. The cylindrical housing contains two immiscible liquids of different refractive indices. One of the liquids is electrically conducting, for example an aqueous salt solution, the other is insulating, for example a non-polar oil. If both liquids have equal densities, the shape of the meniscus is perfectly spherical, independent of orientation and rather insensitive to external vibrations and shocks. The glass cylinder is coated with a transparent electrode in order to observe the oil/water interface from the side. The inside of the cylinder is coated with a hydrophobic insulator. The counter electrode is in direct contact with the conducting liquid.

!"

!" $

" !" %

A

B

&

θ

# !

'

Fig. 1 (A) Schematic cross section of a liquid-based variable lens in a cylindrical glass housing. The transparent electrodes are formed of 50 nm indium tin oxide deposited by sputtering. The insulator is a 3-µm parylene-N film, formed by chemical vapor deposition. The 10-nm hydrophobic top coating is a soluble fluoropolymer (AF1600, supplied by Dupont), deposited by dip coating. The top and bottom glass plates are glued onto the glass cylinder with epoxy glue. (B) When a voltage is applied, charges accumulate in the wall electrode and opposite charges collect near the solid/liquid interface in the conducting liquid. The resulting electrostatic force effectively lowers the solid-liquid interfacial tension and with that the contact angle θ.

Application of a voltage between the electrodes (Fig. 1B) results in an electric field across the insulator, which effectively lowers the interfacial tension

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Voltage Reduction in Electrowetting-on-Dielectric

between the conductive liquid and the insulator. The resulting change in contact angle θ of the conducting liquid with the wall can be described by combining Lippmann’s equation with Young’s equation into the so-called LipmannYoung equation:

cosθ =

( , ε γ. * −γ. ) +( V γ )-* γ )+* d

for −1 ≤ cos θ ≤ 1

(1),

where ε denotes the dielectric constant of the insulating film, d its thickness, V the applied voltage, γci the interfacial tension between both liquids, γwc between the wall and the conducting liquid and γwi between the wall and the insulating liquid. Figure 2 shows three video frames of a liquid lens at various voltages. A

2LO

C

B

:DWHU

0V

100V

120V

Figure 2. (A) to (C) Video frames of a 6-mm diameter lens taken at voltages of approximately 0 V, 100 V and 120 V.

Figure 2A shows the lens in the off state. For most combinations of liquids, the contact angle is around 180° in this state. Often the ratio of surface tensions (γwi-γwc)/γci is slightly smaller than –1, so that a certain threshold voltage is required to change the contact angle. Figure 2C shows the smallest possible contact angle, which is for this particular lens approximately 60°. Higher voltages lead to saturation effects. III. OPTIONS FOR VOLTAGE REDUCTION Equation (1) can be written as

V =

2dγ -/ 0 γ1 0 −γ1 / (cosθ − ) γ +/ 0 ε

(2).

If the lens is used for focusing purposes, it is usually sufficient to use only a small part of the total range. For zoom cameras (two variable lenses in series) with a high zoom factor, a much larger part of the range must be used. In this article, we assume that the meniscus has an initial contact angle of 180° (i.e. (γwi-γwc)/γci= -1) and we want to switch it to an angle of 90°. In other words: we

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want to switch from half a sphere to flat, so from the situation in figure 2A to the situation in figure 2B. Equation (2) shows that this would require a voltage Vflat, given by:

V4 5 6%7 =

2dγ 2+3 ε

(3).

Any other switching range would just lead to the same expression multiplied by a different constant. We will use (3) to study the options for voltage reduction. At first sight, and as is often thought, it seems that the voltage scales with the square root of ε, d and γci. A closer look, however, teaches that reality is more complicated. A. Layer thickness Equation (3) shows that a thinner insulating layer leads to a lower voltage. However, a thinner layer also leads to a larger electric field

E=

V d

(4)

inside the layer. Combination of (3) and (4) shows this effect:

E : ; < = =

2γ 8+9 dε

(5),

where Eflat denotes the electric field that arises inside the insulating layer when the meniscus is switched from half a sphere to flat. Below a certain layer thickness dmin, flat, the electric field will exceed the dielectric strength E breakdown, leading to dielectric breakdown. This minimum layer thickness follows from (5):

d QRM JS N O C%P =

2γ L+M > A ? εE @ BDC E"FHG+IKJ

(6).

Saturation of the electrowetting effect, which is probably field-driven, usually occurs at electric fields well below the dielectric strength. For example, the contact angle of the lens in figure 2 saturates at a voltage of 120 V (figure 2C). The electric field in the parylene-N layer is then 120V/3µm=40V/µm, while the dielectric strength of parylene-N is approximately 280V/µm. One explanation for this difference is the fact that the local field near the meniscus edge is much larger than the average field given by (4), due to edge effects. This is

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Voltage Reduction in Electrowetting-on-Dielectric

also the reason for the fact that breakdown occurs well below 280V/µm. As the saturation effect is not yet well understood, we will only look at the dielectric strength here. However, the equations we derive might also be used with respect to saturation, although in that case a lower value for the maximum allowable electric field should be used. The graph in figure 3 illustrates the existence of a minimum layer thickness.

j f

i g

=

}~

if hg

V =

hf g f

£¤ +%¥¦A¡+§¨¡H+%H¤% ¢"-©$¡+¤%% " © k

kml kn

kml kpo

2dγ $ ε

%" ¡A%$ ¢ d min,flat kKl krq

uwv xzy|{

kKl kps

klt

Figure 3. The curved line indicates the voltage for which the meniscus is flat, as a function of the parylene-N layer thickness. The straight line indicates the voltage above which dielectric breakdown occurs. The intersection of both lines indicates the minimum layer thickness that is required to obtain a flat meniscus without dielectric breakdown.

In figure 3 we use γci=0.045N/m (for silicone oil and water), Ebreak-12 F/m, with εr the down=280V/µm (for parylene-N) and ε=εrε0 = 2.65⋅8.854⋅10 relative dielectric constant and ε0 the vacuum permittivity. We assumed that the thin AF-1600 topcoat has no influence on the electrical properties. The intersection of the two lines can be calculated with (6) and occurs for dmin,flat=0.049µm. The corresponding value of V=13.7V is the lowest possible voltage for which a flat meniscus can be obtained using a parylene-N layer. A general expression for the lowest possible voltage Vmin,flat for which a flat meniscus can be obtained, is reached by combining (3) and (6):

Vc ^ de _ ` a b =

2γ ]-^ A T εE U%V-WDXHYAZ+[\

(7).

Note that for a good and reliably working device it is preferred to stay well above the minimum required layer thickness to avoid charging or time dependent breakdown of the insulating coating.


33

B. Interfacial tension Equation (7) shows that for a smaller interfacial tensionγci between the conducting and insulating liquid, a lower voltage can be obtained. Equation (3) also shows this effect, but in (3) the dependency goes with the square root, whereas in (7) it is linear. The difference is caused by the fact that in (3) d is kept constant, whereas in (7) d decreases with decreasing γci. For a decreasing γci causes a lower voltage according to (3) and this in turn allows for a decrease of d, which lowers the voltage even further. The liquid/liquid interfacial tension can be lowered by using different combinations of liquids. However, these liquids may not have the desired properties. An alternative is the use of surfactants. We added sodium dodecylsulphate (3.9⋅10-4M) to the 0.1M KCl solution that was used in figure 2 and did not change the silicone oil. It was observed that the voltage required to obtain a flat meniscus decreased from 100V to 60V, for the same layer thickness. This indicates that, according to (3), γci dropped by a factor of 0.36 from 0.045N/m to 0.016V/m. Thus, according to (7), it should be possible to reduce the minimum voltage for which the meniscus is flat by a factor of 0.36 by adding sodium dodecylsulphate and simultaneously reducing the layer thickness by a factor of 0.36. We assumed that the off-state contact angle was 180°, i.e. (γwi-γwc)/γci=-1. With a smaller value for γci, this ratio decreases. However, the surfactant does not only act between the two liquids, but it will also act between the liquids and the wall. We did not observe a change in initial contact angle or threshold voltage, which indicates that γwc was reduced by approximately the same factor as γci, for γwi εr2, E2>>E1. If the dielectric strengths of both materials are comparable, it is clear that E2 will be the first to reach its breakdown value. The topcoat will break down at a much lower voltage than the insulator. Combined with (7) we can therefore conclude that

VÒRÎ ÊË Ï Ð Å Ñ =

2γ Í-Î Ã Ì Á ε ε EÂHÃ Ä+Å%ÆÇHÈ+É¨ÊË Á

(11).

Equation (11) shows, when compared with (7), that the use of an insulator with a high dielectric constant does not change the value of the lowest voltage for which the meniscus is flat. This result is in contrast to what is generally believed. For instance, Moon et al. report in ‘Low voltage electrowetting-ondielectric’ [7] that they could drastically decrease the voltage by using high dielectric constant materials. They coated a 70-nm layer of barium strontium titanate (BST, εr=180, Ebreakdown=300V/µm) with a 20-nm layer of amorphous  fluoropolymer (Teflon AF, εr=1.9, Ebreakdown=200V/µm). They applied volt-

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ages over the stack ranging from 0V to 25V. The voltages were applied for less than 1 s to prevent time dependent breakdown of the BST. However, if we calculate the electric fields, we see that the BST is not the problem. For the electric field E1 in the BST layer and the field E2 in the fluoropolymer layer applies

E Ô d Ô + EÓ d Ó = V

(12).

Combination with (9) yields

EÖ =

V εÖ dÖ + dÕ εÕ

and E Ó =

V εÓ Ô d + dÓ εÔ

(13).

For a voltage of 25V we obtain E1=13V/µm and E2=1.2⋅103V/µm. Clearly, the field strength in the BST layer is far below the dielectric strength whereas in the fluoropolymer layer it is far above it. What happens is that the fluoropolymer breaks down, while the BST layer stays intact. It may be possible to perform electrowetting for a short time (