Anodic Ta2O5 for CMOS compatible low voltage electrowetting-on-dielectric device fabrication

Abstract

This paper reports a CMOS compatible fabrication procedure that enables ElecroWetting On Dielectric (EWOD) technology to be post-processed on foundry technology. With driving voltages less than 15V it is believed to be the lowest reported driving voltage for any material system compatible with post-processing on integrated circuits. The process architecture uses anodically grown tantalum pentoxide as the pinhole free high dielectric constant insulator with the overlying 16nm layer of Teflon-AF R , which provides the hydrophobic surface upon which droplets can be manipulated. This stack provides a very robust dielectric, which maintains a sufficiently high capacitance per unit area for effective operation at the lower voltage favoured by more standard CMOS technology. The paper demonstrates that the sputtered tantalum layer can be integrated with the aluminium (or copper) interconnect of foundry CMOS processes by standard microfabrication techniques. I. IIn recent years lab-on-a-chip and bio-MEMS systems, which can manipulate and analyse biological fluidic samples in micro-and nano-litre scales, have emerged as a solution for automating repetitive laboratory tasks [1], [2]. Digital microfluidic devices based on technologies such as dielectrophoresis (DEP), electrowetting on dielectrics (EWOD) and surface acoustic waves (SAW) provide a potentially reconfigurable method of obtaining a bio-MEMS system [2], [3]. Of these, EWOD technology is an attractive option that has a low power consumption making it well suited for the design and manufacture of microfluidic systems [2]. EWOD uses surface tension as a driving force, which can be controlled by applying a suitable voltage to an array of electrodes covered by a two layer dielectric.A key parameter in EWOD technology is the driving voltage V. The initial work on electrowetting arrays required driving voltages in the range 80-100V [4]. More recently with a more judicious choice of materials, processes and dielectric thickness, the voltage required to manipulate droplets has been reduced below 15V [4]. However, the temperatures required for the deposition of one of these dielectric layers is well in excess of 450 o C [5] making the process non-compatible with CMOS post-processing. This paper reports a process architecture that matches the driving voltage of [4] while not involving processing temperatures anywhere near 450 o C. A. BackgroundThe technology of the electrocapillary phenomenon has been extensively described elsewhere [1], and will only be discussed briefly. For an EWOD system, the Young-Lippmann equation describes the wetting angle change for a droplet in terms of the applied voltage V, the relative dielectric constant ε r , the liquid-gas surface tension γ lg and the thickness t of the dielectric:Equation (1) identifies the important role played by the dielectric covering the electrodes in determining the driving voltage V required to modify the contact angle θ (40 o typically required for droplet movement). From the Young-Lippmann equation, it is...

Moving-part-free microfluidic systems for lab-on-a-chip

“…Based on the PCB EWOD systems, on-chip sensing has been presented by Luan et al aiming at integrating an optical sensing system with digital microfluidics [50]. Driven by similar motivations, Li et al has reported the integration of EWOD and CMOS technology by post-processing standard foundry-fabricated chips [51], which potentially enables both electrode arrays and sensor arrays to be controlled by integrated addressing CMOS circuit. Figure 7 shows a 1 μl droplet being manipulated on an EWOD electrode array controlled by an 'underneath' circuit fabricated using 100 V CMOS foundry technology.…”

“…A moving droplet on the post-processed EWOD electrode array controlled by an integrated CMOS circuit underneath[51]. Top view of an EWOD device, showing (a) a 1 μl droplet on the reservoir electrode is being pulled out on to a small electrode array, (b) both the reservoir electrode and a small electrode are switched on to pull both sides of the liquid to split it, and (c) 80 nl droplet is dispensed after the splitting[54].…”

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