SU-8 cantilever with integrated pyrolyzed glass-like carbon piezoresistor

Jongmoon Jang,Giovanni Boero,Juergen Brugger,Giulia Panusa

doi:10.1038/s41378-022-00351-9

Abstract

Glass-like carbon (GC) is a nongraphitizing material composed entirely of carbon atoms produced from selected organic polymer resins by controlled pyrolysis in an inert atmosphere. The GC properties are a combination of the properties of glass, ceramic, and graphite, including hardness, low density, low thermal conductivity, high chemical inertness, biocompatibility, high electrical conductivity, and microfabrication process compatibility. Despite these unique properties, the application of GC in mechanical sensors has not been explored thus far. Here, we investigate the electrical, structural, and chemical properties of GC thin films derived from epoxy-based negative photoresist SU-8 pyrolyzed from 700 to 900 °C. In addition, we fabricated microGC piezoresistors pyrolyzed at 700 and 900 °C and integrated them into nonpyrolyzed SU-8 cantilevers to create microelectromechanical systems (MEMS) mechanical sensors. The sensitivities of the GC sensor to strain, force, surface stress, and acceleration are characterized to demonstrate their potential and limits for electromechanical microdevices.

Highlights

The piezoresistive effect is a change in electrical resistivity when a material experiences mechanical strain[1].This effect provides a direct energy/signal conversion from the mechanical to the electrical domain, which is widely used in microelectromechanical systems (MEMS)based sensors, including pressure sensors[2], accelerometers[3], force sensors[4], tactile sensors[5], and flow sensors[6]
Due to its lower Young’s modulus and microfabrication versatility[45], SU-8 has often been applied to MEMS devices such as accelerometers[11], atomic force microscopy (AFM) cantilevers[46], and acoustic sensors[47] as an alternative to harder materials such as silicon
The material properties were evaluated for Glass-like carbon (GC) thin films prepared by pyrolysis at 700– 900 °C for 1 h using a 1-μm-thick cured SU-8 photoresist as the precursor on the Si wafer

Summary

Introduction

The piezoresistive effect is a change in electrical resistivity when a material experiences mechanical strain[1].This effect provides a direct energy/signal conversion from the mechanical to the electrical domain, which is widely used in microelectromechanical systems (MEMS)based sensors, including pressure sensors[2], accelerometers[3], force sensors[4], tactile sensors[5], and flow sensors[6]. The piezoresistive effect is a change in electrical resistivity when a material experiences mechanical strain[1]. The GF is more than one order of magnitude higher than the GF in metals (e.g., p-type Si has a GF of ~100)[7]. These large GFs are caused by the large change of the electrical resistivity (Δρ), which, in turn, is due to the variation of the carrier density and of the mobility induced by the deformation of the band structure[8,9]. In polymer nanocomposites consisting of conductive nanoparticles in a polymer matrix, GF is determined by the tunnelingpercolation between nanoparticles and the high flexibility of the polymer[10,11,12]

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