Maskless Droplet-on-Demand (DoD) Systems
Jeffrey Bokor and Nathan Emley
We develop a monolithic, MEMS-actuated device able to generate sub-micron diameter (femtoliter) fluidic droplets. Such droplet-on-demand (DoD) devices are important for maskless lithography, rapid prototyping, and spotting applications. A suspended membrane flexure actuator was chosen over thermal-bubble methods because it can produce higher and better-controlled pressures, which is critical for sub-micron droplet formation. The MEMS actuation also introduces no thermal damage to the fluid solution, which is important for biological fluids and nanoparticle suspensions. Advances in sol-gel-derived lead zirconate titanate (PZT) thin film deposition allow for wafer-scale integration of PZT, a strong piezoelectric material, as the active layer. LPCVD dielectrics serve as structural underlayers and the suspended membrane will be formed by a backside etch of the Si handle. Actuation of the PZT in the transverse or d31-mode, where lateral contraction of the PZT induces a bending of its suspended dielectric underlayers (the flexure plate geometry), can achieve the requisite speed and deflection amplitude. Design of the actuator, orifice, and actuation pulse shape are all predicted to have a significant impact on the dynamics of droplet formation and are thus critical in successfully achieving sub-micron diameter droplet formation.
Figure 1: Schematic of the membrane actuator, which consists of passive, structural layers (LPCVD nitrides, oxide) and active layers (Pt/PZT/Pt)
Figure 2: Cross-sectional SEM image of a backside through-etch of the Si handle
Figure 3: ANSYS finite element modeling of the multilayered membrane. Fundamental (i.e., uniform) resonance frequency as a function of membrane radius for different clamping and in-plane stress conditions