Study on the Piezoresistivity of Cr-Doped V₂O₃ Thin Film for MEMS Sensor Applications

Academic Background

Piezoresistive microelectromechanical systems (MEMS) sensors are devices that utilize the piezoresistive effect of a material to convert stress changes, induced by the physical property being observed, into resistance changes. These sensors, such as pressure sensors, accelerometers, and strain sensors, are widely used in automotive, aerospace, biomedical, and research applications. Currently, most piezoresistive sensors use doped silicon as the piezoresistive material, which, despite its ease of integration and cost-effectiveness, has limited piezoresistive effects, with a longitudinal gauge factor (GF) typically below 120. To enhance sensor performance, researchers have been exploring new piezoresistive materials. Vanadium oxide materials have attracted significant attention due to their unique piezoresistive properties, particularly Cr-doped V₂O₃ thin films, which exhibit significant resistance changes at room temperature through strain control, showing potential as a piezoresistive material.

Paper Source

This paper was co-authored by Michiel Gidts, Wei-Fan Hsu, Maria Recaman Payo, Shaswat Kushwaha, Frederik Ceyssens, Dominiek Reynaerts, Jean-Pierre Locquet, Michael Kraft, and Chen Wang, from the Micro and Nanosystems Laboratory and the Functional Oxides Coating Center at KU Leuven, Belgium. The paper was published in 2024 in the journal Microsystems & Nanoengineering.

Research Process

1. Thin Film Growth and Structural Characterization

Cr-doped V₂O₃ thin films were grown on sapphire substrates using molecular beam epitaxy (MBE). The crystal structure of the films was characterized using high-resolution X-ray diffraction (XRD) and reflection high-energy electron diffraction (RHEED) to ensure epitaxial growth quality.

2. Fabrication of Piezoresistive Devices

After film growth, piezoresistive devices measuring 500 μm in length and 25 μm in width were fabricated using reactive ion etching (RIE). Metal contacts were formed by depositing chromium and gold via magnetron sputtering, and the sapphire substrate was diced into rectangular samples measuring 38 mm in length and 5.3 mm in width.

3. Electrical and Mechanical Testing

The resistance changes of Cr-doped V₂O₃ thin film piezoresistors with different orientations were measured using a four-point bending test setup. The strain distribution in the sample was simulated using COMSOL Multiphysics software to validate the experimental setup.

4. Design and Characterization of Pressure Sensors

A micromachined pressure sensor based on Cr-doped V₂O₃ thin film piezoresistors was designed and fabricated. The sensor’s output voltage was measured under various pressures and temperatures to evaluate its sensitivity, offset, and temperature coefficients.

Main Results

1. Piezoresistive Effect

Cr-doped V₂O₃ thin film piezoresistors exhibited significant piezoresistive effects at room temperature, with a longitudinal gauge factor (GFₗ) of 222 and a transverse gauge factor (GFₜ) of 217. Piezoresistors with different orientations showed similar resistance changes under strain, indicating isotropic piezoresistive coefficients.

2. Pressure Sensor Performance

The pressure sensor based on Cr-doped V₂O₃ thin film exhibited a sensitivity of 21.81 mV/V/bar, an offset of -25.73 mV/V, a temperature coefficient of sensitivity of -0.076 mV/V/bar/°C, and a temperature coefficient of offset of 0.182 mV/V/°C at 20°C. The sensor’s linear output and low hysteresis suggest its suitability for practical applications.

Conclusion

This study demonstrates that Cr-doped V₂O₃ thin films exhibit significant piezoresistive effects through strain-induced Mott metal-insulator transitions, with isotropic piezoresistive coefficients. This property makes the material highly promising for MEMS sensor applications, particularly in pressure sensors. The research provides new insights into the application of Mott transition-based piezoresistive materials in MEMS sensors.

Research Highlights

1. High Gauge Factor: The longitudinal and transverse gauge factors of Cr-doped V₂O₃ thin films exceed 200, significantly higher than traditional doped silicon materials.
2. Isotropic Piezoresistive Effect: The piezoresistive coefficients are independent of device orientation, simplifying sensor design and layout.
3. Application of Mott Transition: Strain-induced Mott metal-insulator transitions result in significant resistance changes, opening new avenues for research on novel piezoresistive materials.
4. Practical Application Validation: A micromachined pressure sensor based on Cr-doped V₂O₃ thin film was successfully designed and fabricated, demonstrating its feasibility in practical applications.

Research Significance

This study not only provides high-performance piezoresistive materials for MEMS sensors but also opens new avenues for research on Mott transition-based piezoresistive materials. The unique piezoresistive properties of Cr-doped V₂O₃ thin films have the potential to advance sensor technology in fields such as healthcare, automotive, and aerospace.