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The response of micromachines to the external actions is typically affected by a scattering, which is on its own induced by their microstructure and by stages of the microfabrication process. The progressive reduction in size of the mechanical components, forced by a path towards (further) miniaturization, has recently enhanced the outcomes of the aforementioned scattering, and provided a burst in research activities to address issues linked to its assessment [1,2]. In this work, we discuss the features of an on-chip testing device that we purposely designed to efficiently estimate the two major sources of scattering affecting inertial, polysilicon-based micromachines: the morphology of the silicon film constituting the movable parts of the device, and the etch defect or overetch induced by microfabrication. The coupled electro-mechanical behavior of the statically determinate movable (micro)structure of the on-chip device has been modelled via beam bending theory [3], within which the aforementioned sources of scattering have been accounted for through local fluctuating fields in the compliant part of the structure itself, namely the supporting spring. The proposed stochastic model is shown to outperform former ones available in the literature [4,5], which neglected the simultaneous and interacting effects of the two mentioned sources on the measure response. The model can fully catch the scattering in the C-V plots up to pull-in, hence also in the nonlinear working regime of the device. References [1] Zhu, J.; Liu, X.; Shi, Q.; He, T.; Sun, Z.; Guo, X.; Liu, W.; Sulaiman, O.B.; Dong, B.; Lee, C. Development Trends and Perspectives of Future Sensors and MEMS/NEMS. Micromachines 2020, 11. [2] Molina, J.P.Q.; Rosafalco, L.; Mariani, S. Stochastic Mechanical Characterization of Polysilicon MEMS: A Deep Learning Approach. Proceedings 2020, 42. [3] Mirzazadeh, R.; Eftekhar Azam, S.; Mariani, S. Micromechanical Characterization of Polysilicon Films through On-Chip Tests. Sensors 2016, 16, 1191. [4] Mirzazadeh, R.; Ghisi, A.; Mariani, S. Statistical Investigation of the Mechanical and Geometrical Properties of Polysilicon Films through On-Chip Tests. Micromachines 2018, 9, 53. [5] Ghisi, A.; Mariani, S. Effect of imperfections due to material heterogeneity on the offset of polysilicon MEMS structures. Sensors 2019, 19, 3256.