Study on the piezoresistivity of Cr-doped V2O3 thin film for MEMS sensor applications

Abstract

Cr-doped V₂O₃ thin film shows a huge resistivity change with controlled epitaxial strain at room temperature as a result of a gradual Mott metal-insulator phase transition with strain. This novel piezoresistive transduction principle makes Cr-doped V₂O₃ thin film an appealing piezoresistive material. To investigate the piezoresistivity of Cr-doped V₂O₃ thin film for implementation in MEMS sensor applications, the resistance change of differently orientated Cr-doped V₂O₃ thin film piezoresistors with external strain change was measured. With a longitudinal gauge factor of 222 and a transversal gauge factor of 217 at room temperature, isotropic piezoresistivity coefficients were discovered. This results in a significant orientation-independent resistance change with stress for Cr-doped V₂O₃ thin film piezoresistors, potentially useful for new sensor applications. To demonstrate the integration of this new piezoresistive material in sensor applications, a micromachined pressure sensor with Cr-doped V₂O₃ thin film piezoresistors was designed, fabricated and characterized. At 20 °C, a sensitivity, offset, temperature coefficient of sensitivity and temperature coefficient of offset of 21.81 mV/V/bar, -25.73 mV/V, -0.076 mV/V/bar/°C and 0.182 mV/V/°C, respectively, were measured. This work paves the way for further research on this promising piezoresistive transduction principle for use in MEMS sensor applications.

Fig. 1. Characterisation of Cr-doped V₂O₃ TF.

a Corundum crystal structure (space group: $\mathrm{R}\stackrel{\circledR }{3}\mathrm{c}$) with a the in-plane lattice parameter. b RHEED image taken at room T before and after deposition. c XRD ϕ scan. d Phase diagram of the ${\left({\mathrm{V}}_{1-\mathrm{x}}{\mathrm{M}}_{\mathrm{x}}\right)}_2{\mathrm{O}}_3$ system for thin films from ref.

Fig. 2. Fabrication steps.

Illustration of the fabrication steps of Cr-doped V₂O₃ TF pressure sensor and bending sample. 1a, 1b Cr-doped V₂O₃ TF deposition and etching on sapphire substrate. 2a, 2b Deposition and lift-off of Cr and Au metal contacts. 3a Milling of sapphire to fabricate sapphire membrane

Fig. 3. Cr-doped V₂O₃ TF piezoresistors.

Picture of fabricated Cr-doped V₂O₃ TF piezoresistors on a sapphire beam sample with close-up of the differently orientated piezoresistors

Fig. 4. Doped Si resistance change with stress change.

Measured resistance change with stress change for a doped silicon piezoresistor stressed by four-point bending

Fig. 5. Cr-doped V₂O₃ TF electrical and stress measurements.

a I-V curve of Cr-doped V₂O₃ TF piezoresistors. b Resistance of Cr-doped V₂O₃ TF piezoresistors with applied strain. c Resistance change of Cr-doped V₂O₃ TF piezoresistors with positive applied strain. d Cr-doped V₂O₃ TF piezoresistivity coefficients for different orientations of the piezoresistor in the XY plane of the crystallographic framework. e Resistance change of Cr-doped V₂O₃ TF piezoresistor M5 with increasing and decreasing strain. f Resistance change of Cr-doped V₂O₃ TF piezoresistor M5 with increasing and decreasing temperature

Fig. 6. Cr-doped V₂O₃ sapphire pressure sensor.

a Simulated strain ε_x + ε_y on the membrane for a pressure of 2 bar with superimposed piezoresistors and metallization for visualization. b Picture of fabricated Cr-doped V₂O₃ TF piezoresistors on a sapphire beam sample with close-up of the differently orientated piezoresistors

Fig. 7. Cr-doped V₂O₃ sapphire pressure sensor measurement result.

a Measured output voltage in mV/V of the Wheatstone bridge. b Linearity error in % span with different temperatures and pressures. c Repeated sensitivity measurements at different temperatures. d Repeated offset measurements at different temperatures. e Measured sensitivity when cycling the pressure and temperature. f Measured offset when cycling the pressure and temperature