Subsurface Imaging of Microfabricated Silicon Membrane–Cavity Structures Using Near‐Infrared Interferometry

To characterize microfabricated resonators, fabricated membrane or cavity structures are often destructively examined with high‐resolution microscopy, although such an approach may cause structural or morphological changes. Herein, nondestructive and direct measurement of the thickness of the suspended membrane as well as the height of the underlying cavity in silicon‐on‐nothing (SON) structures by custom‐built near‐infrared (NIR) interferometry are demonstrated. While positions of the surface and all buried interfaces of the as‐fabricated single‐layered SON are detected by NIR interferograms, positions of the surface and the first three interfaces (i.e., except the deepest interface) of the as‐fabricated double‐layered SON structure are detected. While the submicrometer‐thick membrane thickness and the cavity height of the polished single‐layered SON are also measured, smoothened surface roughness allows multiply reflected rays to show up and counterintuitively complicates the detection of the buried interface. The NIR interferometry results in measurement errors ranging from 0.7% to 3.3% for the membrane thickness and those ranging from 2.2% to 13% for the cavity height, compared to scanning electron microscopy. It is thus anticipated that a NIR interferometry can be a versatile tool for characterizing a variety of microdevices.