Annealing Effect of High Dielectric Material for Low Voltage Electrowetting on Dielectric ( EWOD )

In this paper, the high dielectric constants for Ta2O5 (~18.8) and Nb2O5 (~25.5) were deposited by a RF reactive magnetron sputtering and respectively annealed at 700 °C and 400 °C O2 ambiance for 30 min in a conventional furnace. The purpose of this study is to optimize the annealing condition (various temperatures at N2 or O2 ambiance) of the high-dielectric-constant Ta2O5 and Nb2O5 films deposited by RF reactive magnetron sputtering to enhance the dielectric constant of those films to further lower the operating voltage. Based on the results, an electrowetting optical deflector (EOD) filled with the water (1% sodium dodecyl sulfate (SDS)) and dodecane was fabricated and tested, and the contact angle of the inclined liquid surface on the left and right sidewall can be varied about 70° at 9 V operating voltage. This study provides a practical way to fabricate a high dielectric constant layer for low voltage electrowetting on dielectric (EWOD) application.


Introduction
Electrowetting phenomenon was first exploited by Lippmann (1875).By varying the voltage between the electrolyte droplet and the substrate, the contact angle of droplet can be modulated.Due to the electrolysis effect, the room to manipulate the contact angle of droplet is very limited.Till Berge (1993) inserted a thin insulating layer between the electrolyte droplet and the electrode to eliminate the electrolysis problem, the contact angle change can be dramatically increased at a large voltage.This improved technology was so called EWOD.Since then, it initiated an explosive growth in electrowetting research, especially in the field of optics (Mugele & Baret, 2005;Kuiper, Hendriks, Hayes, Feenstra, & Baken, 2005;Hou, Zhang, Smith, Yang, & Heikenfeld, 2010;Ceyssensa et al., 2013).
Many studies have used SiO 2 with a low dielectric-constant of about 3.8 as the insulating layer, leading to a large operating voltage of several tens of volts (Smith, Abeysinghe, Haus, & Heikenfeld, 2006;Papathanasiou, Papaioannou, & Boudouvis, 2008;Cho, Fan, Moon, & Kim, 2002;Cho, Moon, & Kim, 2003).The larger operating voltage will cause electrical breakdown and reliability.Thus, in the electrowetting optics application, the low operating voltage is the future trend in order to be compatible with commercial electronic components, reliability and conserve power.Known from Lippmann-Young equation (Mugele & Baret, 2005), decreasing the thickness of the dielectric layer, employing a high dielectric constant material, and minimizing the interfacial surface tension between the electrolyte and the surrounding ambient phase are the three applicable approaches to drop the required operating voltage.However, thinning the dielectric layer tends to induce dielectric breakdown (Berry, Kedzierski, & Abedian, 2006) at high electric field; besides, adding surfactants to the oil-water interface has been proven to slow down the oil-water response time (Roques-Carmes, Palmier, Hayes, & Schlangen, 2005).Therefore, using high dielectric films is the most potential method among the three to achieve the low operating voltage without suffering from other side effects.
A lot of efforts have been dedicated to exploring the deposition of high dielectric constant materials, and promising progress has been reported (Moon, Cho, Garrrell, & Kim, 2002;Li et al., 2008;Chang, Choi, Han, & Pak, 2009;Raj, Dhindsa, Smith, Laughlin, & Heikenfeld, 2009;Lin, Evans, Welch, Hsu, Madison, & Fair, 2010).However, the facilities used in those studies, such as metal-organic chemical vapor deposition (MOCVD) (Moon et al., 2002) and atomic layer deposition (ALD) (Chang, Choi, Han, & Pak, 2009;Raj, Dhindsa, Smith, Laughlin, & Heikenfeld, 2009), are not widely available.As a consequence, some other cost-saving approaches have been suggested for the material deposition.In the dense wavelength division multiplexing (DWDM) system, it required a high dielectric films (e.g., Ta 2 O 5 , Nb 2 O 5 , or TiO 2 ) to fabricate an interference filter.In general, the most common way to deposit these high dielectric layers is by RF reactive magnetron sputtering, and most of the metal oxides can be deposited by using the metal targets and reaction gas mixtures (e.g., O 2 and Ar).
The purpose of this research was to fabricate a high dielectric constant layer to lower the operating voltage.According to Park's findings (Park, Li, Nam, & Rhee, 1999), a high dielectric constant film can be produced by using sputtering and annealing technologies.The purpose of this study is to optimize the annealing condition of the high-dielectric-constant Ta 2 O 5 and Nb 2 O 5 films deposited by RF reactive magnetron sputtering (Lin et al., 2011;Zhou, Luo, Li, & Liu, 2009;Coskun & Demirel, 2013;Lai, Lin, Huang, Gai, & Qu, 2006) to enhance the dielectric constant of those films to further lower the operating voltage.The annealing was taken at various temperatures under N 2 or O 2 ambiance in a conventional furnace.The dielectric constant of the resulting films was deduced from capacitance measurement with an inductance capacitance resistance (LCR) meter, and the film surface morphologies were investigated with scanning electron microscope (SEM) and atomic force microscope (AFM).Finally, an electrowetting optical deflector (EOD) device (Chen & Fu, 2011) consisting of a 200-nm Nb 2 O 5 , layer annealed at 400 °C for 30 min under O 2 ambient was fabricated and tested.The contact angle of the inclined liquid with respective to the EOD sidewalls can be varied up to 70° at the operating voltage of 9 V.

Experimental Procedures
The cleaning procedures of the substrate for dielectric film deposition were briefed as follows.First, the p-type (100) silicon wafers were immersed in the piranha solutions for 5 min to remove organic contamination, and then dipped in the buffered oxide etch (BOE) for 30 sec to strip off the native oxide.Next, the wafers were rinsed with deionized (DI) water with resistivity of ~18 MΩ, were dehydrated on a hotplate at 200 °C for at least 20 min, and were cooled to room temperature.
The deposition of dielectric films was carried out in a RF magnetron sputtering system, and the tantalum and niobium targets of 99.99% purity were employed for the Ta 2 O 5 and Nb 2 O 5 film deposition.To start the sputtering process, the system was first evacuated to a base pressure of 0.67 mPa, followed by the Ar and O 2 gases flow with the rates keeping at 27 sccm and 3 sccm, respectively, corresponding to a total gas pressure of 0.4 Pa.Then, the sputtering power was set to be 300 W, and the substrate temperature was raised to 100 °C.The deposition rates for Ta 2 O 5 and Nb 2 O 5 were 10 and 2 nm/min, respectively.The dielectric layers with thicknesses of 200 nm were prepared for each material, and the thickness of each coated layer was measured with a surface profile meter (AMBIOS XP-1).Finally, these samples were respectively annealed in a conventional N 2 or O 2 furnace using at temperatures ranging from 400 °C to 1000 °C for 30 min.After annealing procedure, the samples were cooled down to room temperature.
The dielectric constant, surface morphologies, and surface roughness for the resulting dielectric films were respectively analyzed by the LCR (Agilent E4980A) meter, SEM (Hitachi S-4800), and AFM (Veeco) before and after annealing.In order to measure dielectric constants, a metal-insulator-semiconductor (MIS) capacitor with a 500 × 500 μm 2 Al pad was fabricated on top of the dielectric layer using photolithography and lift-off process.The p-type Si substrate was positive biased, and the Al pad was negative biased.The dielectric constant of the resulting films was deduced from capacitance measurement at various frequencies (100 Hz ~100 kHz) and bias (50 mV, 250 mV, and 1 V) with the LCR meter.To calculate the dielectric constant, the post-annealing sample thickness was measured by its cross-sectional SEM image.The surface roughness of dielectric films was analyzed by tapping mode AFM, and the scan area was 1 × 1 μm 2 .
An EOD device was fabricated to test the contact angle change of the inclined liquid surface on the sidewalls.The detail fabrication processes have been reported elsewhere (Kuiper et al., 2005;Smith, 2006;Chen et al., 2011).As shown in Figure 1(a), an EOD chamber included two Si sidewalls (coated with a composite dielectric layer) as electrodes, a transparent indium tin oxide (ITO) glass spacer (~3 mm wide) in the bottom, a top glass plate, and two front and back sealing glass plates.The composite dielectric layers was a 200-nm Nb 2 O 5 dielectric layer (annealed at 400 °C O 2 ambiance for 30 min) covered by a 100 nm CYTOP ® hydrophobic fluoropolymer.
After assembling, the EOD chamber was filled with water (containing 1% SDS) and dodecane, and the liquid-liquid interface formed a convex shape (Figure 1(a)).Figure 1(b) shows the schematic of the voltage connections and liquid incline angle measurement system.The experimental flow chart is shown in Figure 2. The EOD's operation required three electrical terminals: two DC voltage sources (V L and V R ) attached to the Si electrodes, and the bottom ITO electrode was electrically grounded.The EOD device was operated in one of the three modes:

Dielectric Constant Characteristics
The thin film deposited by RF reactive magnetron sputtering will form amorphous atoms or ions randomly, and there are dangling bonds and voids.When annealing to certain temperature, the thin film will crystalline and improve crystal imperfections (defect, impurity) to enhance the dielectric constant.To Achieve such a new material could benefit all EWOD devices in terms of lower voltage, east to fabricate and improved reliability.Figure 7 shows the dielectric constants of the optimized films when the measuring frequencies were varied from 100 Hz to 100 kHz, and measuring voltages were varied from 50 mV to 1 V.The results indicate that the dielectric constant decreased as the measuring frequency increased.These findings were in good agreement with those reported (Joshi et al., 1999;Masse, Szymanowski, Zabeida, Amassian, Klemberg-Sapieha, & Martinu, 2006) for Ta 2 O 5 and Nb 2 O 5 .The literature (Shinriki, Nishioka, Ohji, & Mukai, 1989) shows that Ta 2 O 5 and Nb 2 O 5 films crystallize at 650 °C and 450 °C to form a hexagonal structure and enhance the dielectric constant.Besides, in these oxidation processes (annealing temperature ≧900 °C), the dielectric constant was reduced due to the growth of a SiO 2 film, which has a small dielectric constant (3.8), between the dielectric film and the silicon substrate.This can explain the reason that dielectric constant decreased as the annealing temperature increased.
Figure 7.The dielectric constant as a function of measuring frequencies with O 2 annealing temperature at (a) 700 °C Ta 2 O 5 ; and (b) 700 °C Nb 2 O 5 .The measuring voltages were varied from 50 mV to 1 V The surface roughness for as deposited Ta 2 O 5 and Nb 2 O 5 dielectric films was very small (≦0.51 nm); however, when the annealing temperature was gradually increased, the surface of the dielectric layers began to form grain boundary (see Figures 8(b) and 10(b)) and the surface roughness was increased.In the Ta 2 O 5 dielectric film, the grain boundary did not form much as the anneal temperatures were below 650 °C; while, when the annealing temperature was above 450 °C, the Nb 2 O 5 dielectric film formed a distinct grain boundary (Masse et al., 2006).The surface roughness for the highest dielectric constant conditions of Ta 2 O 5 (at 700 °C O 2 annealing) and Nb 2 O 5 (at 400 °C O 2 annealing) was 0.65 nm and 1.15 nm, respectively.The surface roughness affects the contact angle and contact angle hysteresis (the difference between forward and backward contact angle).In our case, the low surface roughness is needed to avoid reliability problems.Experimental results show that the contact angle of the inclined liquid surface on the two EOD sidewalls can vary about 70° at 9 V operating in a dodecane/water/Cytop ® /Nb 2 O 5 system, and which was reduced 2 V operating voltage compared to our previous study (Ta 2 O 5 without annealing treatment) (Chen et al., 2011).

Surface Morphology Features
Due to the contact angle saturation phenomenon, the contact angle of the inclined liquid surface was saturated at 70°.In future work, applying an AC operating voltage (Nanayakkara et al., 2010) and a high quality dielectric film (pinhole free) to avoid charges trapping (Verheijen & Prins, 1999) will reduce contact angle saturation.This paper may provide a good reference for the dielectric constant characteristics in the AC operating voltage.

Conclusions
The high dielectric constant layers (Ta 2 O 5 and Nb 2 O 5 ) were deposited on the silicon substrate by a RF reactive magnetron sputtering and annealed at various temperatures under N 2 or O 2 ambiance in a conventional furnace.
The dielectric constant and surface roughness of the dielectric layers before and after various annealing treatments were studied by an LCR meter, SEM and AFM instruments.The as-deposited dielectric films have an amorphous structure, and the surface roughness is very small (≦0.51 nm).However, when the annealing temperature was gradually increased, the surface of the dielectric layers began to form grain boundary and the surface roughness becomes larger.Experimental results show that annealed in the N 2 ambiance did not enhance the dielectric constant than as-deposited dielectric films, but annealed in the O 2 ambiance can enhance the dielectric constants below certain temperature.
This study gets the high dielectric constants for Ta 2 O 5 (~18.8) and Nb 2 O 5 (~25.5)deposited by a RF reactive magnetron sputtering and respectively annealed at 700 °C and 400 °C O 2 ambiance for 30 min in a conventional furnace.Moreover, we show that the contact angle of EOD device can change 70° for a dodecane/water/Cytop ® /Nb 2 O 5 /Si system with an applied voltage as low as 9 V.

Figure 1 .
Figure 1.The schematic of (a) a basic EOD device structure; and (b) the three operating modes of an EOD device: V L =V R , V L >V R , and V L <V R from left to right

Figures 3 Figure 3 .
Figures 3 and 4 illustrate the dielectric constant as a function of various N 2 and O 2 ambient annealing temperatures for Ta 2 O 5 and Nb 2 O 5 .The measuring frequency was fixed at 100 Hz, and measuring voltages were varied from 50 mV to 1 V.The results show that the dielectric constant was found to slightly vary as the measuring voltage increased.This was due to the reason that the MIS capacitance was biased at the accumulation mode voltages.

Figure 5 .
Figure 5. Ta 2 O 5 dielectric constant as a function of (a) N 2 ; and (b) O 2 annealing temperatures.The measuring voltage was fixed at 1 V, and measuring frequencies were varied from 100 Hz to 100 kHz.The zero annealing temperature indicates the as-deposited samples

Figures 8 -
Figures 8-11 show the SEM and AFM images of the dielectric film for as deposited and annealed at various temperatures.As shown in Figures 8(a) and 10(a), the surface morphology of the Ta 2 O 5 and Nb 2 O 5 films was smooth and no defects.Figure 12 displays the average roughness values as a function of N 2 and O 2 annealing temperatures for Ta 2 O 5 and Nb 2 O 5 dielectric films.The results show that the surface roughness rose as the annealing temperature increased.The surface roughness change for Ta 2 O 5 (0.42 nm ~ 1.93 nm) was much smaller than the Nb 2 O 5 (0.51 nm ~ 9.06 nm) under various N 2 and O 2 annealing temperatures.

Figure 8 .Figure 12 .
Figure 8.The SEM images show the surface morphology of the Ta 2 O 5 film for (a) as deposited, and with N 2 annealing at (b) 400 °C; (c) 600 °C (d) 700 °C; (e) 800 °C; and (f) 1000 °C.The inset picture is the AFM image with 1μm × 1μm area

Figure 13 .
Figure 13.Side view photographs of the EOD device (dodecane and 1%SDS water) with (a) V L= V R = 0 V; (b) V L = V R = 7 V; (c) V L = 9 V, V R = 5 V; and (d) V L = 5 V, V R = 9 V.The water is electrically grounded(Video,  4.11 MB)