MEMS cantilever sensor for non-destructive metrology within high-aspect-ratio micro holes

Tactile metrology with deep and narrow micro holes was addressed using extremely slender piezoresistive micro cantilever sensors. Linear strain–displacement characteristics were observed with this sensor operated under transversal and axial loading. From noise, non-linearity and repeatability measurements the resolution and uncertainty of the cantilever sensors were determined to few nm and few tens of nm, respectively, within a micron displacement span. Under axial loading buckling of the cantilevers was observed after exceeding the critical limit of an Euler beam under the boundary conditions of a clamped-pinned beam. The cantilevers typically survived displacements well above the buckling limit, i.e., fracture of 3-mm long cantilevers was only observed at displacements of more than 200 μm. The feasibility of the cantilever as an active 1D touch probe for high-aspect-ratio blind holes was demonstrated at a dry-etched silicon high-aspect-ratio microstructure. As an application example with a high-volume product we investigated the form and roughness of diesel injector nozzle spray holes.