Flexible MEMS-based platform for high-strain characterization of silicon and vanadium dioxide devices
(2026)
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- (en) In the recent years, the field of strain sensing shifted from the development of conventional strain gauges to the engineering of novel, multifunctional sensors. Strain sensors found new application domains in smart packaging, wearable sensors and health monitoring that require conformable gauges sensitive to multiple physical stimuli in addition to strain such as temperature. Research initially went toward the use of organic piezoresistive materials that can be rendered multifunctional and propose simple read-out circuitry. However, they have limitations in terms of long-term reliability, miniaturisation and output signal quality. To try and cope with these issues, conventional inorganic materials such as silicon or germanium were fabricated onto flexible substrates but do not propose multifunctional sensing. Vanadium dioxide, VO2, is a promising candidate to fill this need since the VO2 crystal is able to transition between different crystallographic states when stimulated. Depending on the magnitude of the stimulation, it can either partially transition from an M1 to an M2 state or completely transition from an insulating M1 state to a metallic R one. These transitions lead to massive changes in the material properties and can be triggered by mechanical, electrical, thermal, optical and magnetic stimulation, making VO2 multifunctional. Looking into strain, the mechanical stimulation generates a partial M1 to M2 reconfiguration accompanied by a massive change of resistivity with reported gauge factors between 250 and 500. Researchers have tried to integrate vanadium dioxide devices onto a flexible substrate for multifunctional sensing but had to use either low-temperature deposition or complex layer transfer. The first approach gives low-quality vanadium dioxide due to low crystallinity while the second is complex and hard to scale. This work proposes to integrate high-quality, thermally annealed, sputtered p-VO2 on a thin-polyimide membrane to characterise its gauge ⋆ | factor under high-strain. The first chapter proposes a monolithic process for the fabrication of microelectronics on a flexible membrane for high-strain studies. The process integrates single crystal silicon (c-Si) devices with electrical connections on a 3.6 μm-thick and 2.7 mm-diameter PI membrane, without limitation or complex layer transfer. c-Si is used instead of p-VO2 to serve as a first demonstrator. It introduces the methodology and tools used in the rest of the work. The second chapter introduces a Comsol FEM model to study the behavior of the bulged membrane and compute the local strain field inside the c-Si devices. The bulged membrane is observed to enter in a plastic deformation regime and to diverge from the expected cube-root deflection profile. The plastic deformation of the membrane is simulated by the Swift plastic model in Comsol. The Comsol model is fitted on an in situ measurement of the membrane deflection and the local strain field in the c-Si is computed. An asymmetrical, biaxial strain field with a maximum magnitude of 0.36 % of local strain is obtained in the c-Si. The strain field is checked by computing the expected silicon Raman peak-shift using the Ganesan equation and comparing it to an experimental measurement. The third chapter explores the integration of thermally annealed, sputtered p-VO2 on a polyimide membrane. A 2.5 mm-wide and 3.2 μm-thick polyimide membrane carrying a 220 nm-thick layer of p-VO2 is fabricated. The deposition and characterization process is presented. The study shows that p-VO2 requires an adhesion layer to stick to the polyimide. The fabricated sample is used to measure the Young’s modulus of p-VO2 since its value is not readily available in literature. A Young’s modulus of 120 GPa is found. Finally, the fourth chapter fabricates patterned p-VO2 resistors on a 1 mmwide and 2.5 μm-thick polyimide membrane. A biaxial strain of 0.3 % is induced in the p-VO2. A continuous and exponential variation of the gauge factor is observed up to 4180 at 0.2 % of strain. This phenomenon is attributed to the intrinsic piezoresistive effect in VO2 and grain dislocation. The chapter shows that the crystal is subjected to irreversible damage past 0.1 % of strain. These results set clear limits on the use of p-VO2 as a high-strain sensor.
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Lahaye, L. (2026). Flexible MEMS-based platform for high-strain characterization of silicon and vanadium dioxide devices. https://hdl.handle.net/2078.5/280920