Sub-picogram mass detection at 400 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si88.svg"> <mml:msup> <mml:mstyle scriptlevel="0"> <mml:mspace width="0.167em"/> </mml:mstyle> <mml:mrow> <mml:mo>∘</mml:mo> </mml:mrow> </mml:msup> <

This study presents a high-temperature microelectromechanical mass sensor based on mode localization, capable of detecting sub-picogram mass perturbations under a 400 °C heater set-point condition. The device consists of two clamped–clamped 3C-SiC beams mechanically coupled by a linking beam and thermally actuated through integrated platinum microheaters. Owing to the mode localization effect, minute mass variations result in highly amplified changes in the amplitude ratio, accompanied by smaller shifts in the resonance frequencies. To optimize the structural design and improve sensitivity, a six-segment analytical beam model based on a 24 × 24 global matrix formulation was developed, and a parametric analysis was performed to examine the influence of geometric dimensions and coupling conditions on modal response. A Pt-actuated 3C-SiC mode-localized coupled resonator was then designed and fabricated, exhibiting stable dual-mode resonance at both 25 °C and 400 °C. Experimental results show that thermally induced tensile stress enhances the effective stiffness and symmetry of the structure, leading to concurrent upshifts of modal frequencies, reduced mode spacing, and an increase in the quality factor from approximately 220 to 400. Incremental focused ion beam (FIB) milling was used to introduce localized mass perturbations as small as 0.43 pg, achieving subpicogram detection with amplitude-ratio sensitivities increasing from 3.47×103 at 25 °C to 7.56×103 at 400 °C. Unlike conventional single-beam resonators, where frequency-based detection fails at this scale and suffers from thermal drift, the coupled design enables common-mode suppression and high sensitivity detection through amplitude-ratio output. By integrating theoretical modelling, parametric simulation, controlled perturbation, and high-temperature validation, this work establishes a unified framework for mode-localized mass sensing and demonstrates the potential of Pt-actuated 3C-SiC coupled resonators for stable and sensitive operation in extreme thermal environments.