Detailed analysis of defect reduction in electrowetting dielectrics through a two-layer ‘barrier’ approach

a r t i c l e i n f o Low-voltage electrowetting requires a thin dielectric capacitor and field strengths approaching 1 MV/cm. Unlike traditional metal/dielectric/metal capacitors, the conducting electrowetted liquid can electrically propagate through the smallest dielectric defects or pores, even for the best barrier polymers such as Parylenes, leading to catastrophic failure such as electrolysis. A detailed analysis of double layer dielectric systems is shown to provide >100 times reduction in defect density, with >10 cm 2 area exhibiting no dielectric failure at >2 times the required electrowetting voltage. An anodized-Al 2 O 3 /Parylene-HT stack provides electrowetting contact angle modulation down to saturation at 70° at b15 V with breakdown protection to >3 times that voltage. These results build on previous findings on the effect of ion type, liquid type, polymer dielectric choice, electrode material, and provide a next major advance in electrowetting reliability. Electrowetting [1] is well known for its use in applications ranging from optics [2–4], to displays [5,6], to lab-on-chip [7–9]. Reliable electrowetting is commercially proven, a prime example being the Varioptic Artic 316 'liquid lens' product which provides >10 8 switching cycles with a several μm-thick Parylene dielectric and 60 V RMS operation. Low-voltage (b15 V) operation has also been demonstrated by several research groups using higher capacitance dielectrics, as recently reviewed by Liu et al. [10]. However, largely unpublished are the very short operating lifetimes (minutes) and low fabrication yields (>1 catastrophic defect/cm 2) for such attempts. Device degradation often takes on the form of electrolysis (Fig. 1a) or electrochemical attack of the underlying electrode. There are several factors that cause low-voltage electrowetting to be particularly challenging. First, as you decrease the dielectric thickness, the required voltage decreases as only the square root of thickness, causing the required electric field to increase [11]. For example, modulation of contact angle from 180° to 70° with 15 V in an oil/water system of 15 mN/m interfacial tension, requires a Parylene dielectric that is ~150 nm thick, and therefore highly stressed under an electric field of ~1 MV/cm. Further problematic, unlike traditional metal/dielectric/ metal capacitors, the conducting electrowetted liquid can electrically propagate through the dielectric, using even the smallest of defects or pores in the dielectric. In addition, most electrowetting devices require dielectric areas of 1's to 100's of cm 2 area, so b 1 dielectric defect per cm 2 is realistically a minimum …