Printing MEMS: Application of Inkjet Techniques to the Manufacturing of Inertial Accelerometers

Abstract

In the last few years, the manufacturing of microelectromechanical systems (MEMS) by means of innovative tridimensional and bidimensional printing technologies has significantly catalyzed the attention of researchers. Inkjet material deposition, in particular, can become a key enabling technology for the production of polymer-based inertial sensors characterized by low cost, high manufacturing scalability and superior sensitivity. In this paper, a fully inkjet-printed polymeric accelerometer is proposed, and its manufacturing steps are described. The manufacturing challenges connected with the inkjet deposition of SU-8 as a structural material are identified and addressed, resulting in the production of a functional spring-mass sensor. A step-crosslinking process allows optimization of the final shape of the device and limits defects typical of inkjet printing. The resulting device is characterized from a morphological point of view, and its functionality is assessed in performing optical readout. The acceleration range of the optimized device is 0-0.7 g, its resolution is 2 × 10^-3 g and its sensitivity is 6745 nm/g. In general, the work demonstrates the feasibility of polymeric accelerometer production via inkjet printing, and these characteristic parameters demonstrate their potential applicability in a broad range of uses requiring highly accurate acceleration measurements over small displacements.

Keywords: MEMS; SU-8; accelerometer; inkjet printing.

Figure 1

Scheme of the proposed accelerometer (a); FEM simulation of the first normal mode (b); FEM simulation of the internal stress distribution in a device (c); schematic representation of the different manufacturing steps: sacrificial layer deposition (d), SU-8 inkjet deposition (e) and sacrificial layer dissolution (f); OM side view of a device printed without step crosslinking before the dissolution of the sacrificial layer ((g) scale bar = 1 mm); OM side view of a device printed without step crosslinking after the dissolution of the sacrificial layer ((h) scale bar = 1 mm); OM top view of a device printed without step crosslinking after the dissolution of the sacrificial layer ((i) scale bar = 2 mm).

Figure 2

Height profile of the Zn sacrificial layer (a); height profile of an as-printed ANS device (b); height profile of an as-printed AWS device (c); theoretical (green) and experimental (red) profile of the SU-8 layer corresponding to a spring (d).

Figure 3

SEM image of the SU-8 layer ((a) scale bar = 6 μm); AFM image of the SU-8 layer (b); SEM image of the sacrificial Zn layer ((c) scale bar = 4 μm); AFM image of the sacrificial Zn layer (d); SEM image of an ANS device at the end of the manufacturing process ((e) scale bar = 100 μm); high magnification SEM image of an ANS device before removal of the Zn layer ((f) scale bar = 20 μm); SEM image of an ANS device before removal of the Zn layer ((g) scale bar = 100 μm); high magnification SEM image of an ANS device after removal of the Zn layer ((h) scale bar = 40 μm); SEM image of an AWS device after removal of the Zn layer ((i) scale bar = 100 μm).

Figure 4

Resonance behavior of an ANS device (a); resonance behavior of an AWS device (b); FEM simulation of a device characterized by a realistic morphology (c); comparison between the resonance frequencies of the AWS and ANS devices with their theoretically predicted FEM values (d).

Figure 5

Ring-down experiment for an AWS device (a); ring-down experiment for an ANS device (b); calibration curve for an ANS device (c); calibration curve for an AWS device; linear fit is displayed as a red straight line (d).