Engineering polymer MEMS using combined microfluidic pervaporation and micro-molding

Abstract

In view of the extensive increase of flexible devices and wearable electronics, the development of polymer micro-electro-mechanical systems (MEMS) is becoming more and more important since their potential to meet the multiple needs for sensing applications in flexible electronics is now clearly established. Nevertheless, polymer micromachining for MEMS applications is not yet as mature as its silicon counterpart, and innovative microfabrication techniques are still expected. We show in the present work an emerging and versatile microfabrication method to produce arbitrary organic, spatially resolved multilayer micro-structures, starting from dilute inks, and with possibly a large choice of materials. This approach consists in extending classical microfluidic pervaporation combined with MIcro-Molding In Capillaries. To illustrate the potential of this technique, bilayer polymer double-clamped resonators with integrated piezoresistive readout have been fabricated, characterized, and applied to humidity sensing. The present work opens new opportunities for the conception and integration of polymers in MEMS.

Fig. 1. Fabrication process flow of multilayer polymer micro-structures.

a (Left) Reversible sealing of dead-end channels embedded in a PDMS mold by a PDMS membrane. Typical dimensions are h = 10–50 µm, e = 10–50 µm, and w = 50–1000 µm. (Right) Transverse view evidencing the solvent pervaporation flux through the PDMS membrane when the channel is filled by a dilute ink (blue). b (Left) schematic 3D view of the pervaporation-induced flow for a single channel (blue arrow). (Right) corresponding schematic pervaporation-induced growth of a material (red). c (Left) removal of the PDMS stamp from the membrane. (Right) the resulting micro-materials are rigid enough to be handled manually, as shown by the SEM image of a simple PVA-CNT beam in which we made a knot (width 150 µm, thickness h = 30 µm). d Alignment of a second structured PDMS mold with the material still embedded in the first block (red). Filling of the channel using a photo-curable polymer (black) makes it possible to fabricate a second layer after UV polymerization (MIMIC process). e (Left) SEM transverse view of an NOA81/PVA-CNT composite bilayer, the total thickness of the structure is H = 115 µm. (Right) Perspective SEM view of the same micro-structure, the width of the main channel is 150 µm

Fig. 2

a Electrical conductivity of PVA-CNT composites as a function of CNT concentration. b SEM image of a double-clamped piezoresistive polymer resonator made by the process illustrated in Figure 1, H = 115 µm, h = 20 µm. c Vibration amplitude and phase of the first, second, and third flexural out-of-plane modes of resonance measured optically. d Zoom on the magnitude of displacement and phase of the first flexural out-of-plane resonant frequency measured optically. Inset: snapshot of the mode shape of the first resonance mode measured optically

Fig. 3

a Schematic view of the half Wheatstone bridge configuration of the piezoresistive transduction. b Piezoresistive detection of the first flexural out-of-plane resonance mode. c Variation of resonant frequency of the first flexural out-of-plane mode measured optically with a laser vibrometer as a function of Joule heating resulting from an applied bias voltage to the bridge resonator. d Relative change of the resonant frequency as a function of relative humidity RH