Inspection of hidden MEMS by an infrared low-coherence interferometric microscope

Micro-electro-mechanical systems (MEMS) are used today in a multitude of applications. The production of MEMS is a fully automated wafer process consisting of deposition, patterning and etching. Especially for safetyrelevant MEMS devices such as airbag or ESP sensors, a 100% inspection is required. After an optical inspection of the MEMS structures, they are sealed and protected by bonding a silicon cap wafer on top. This bonding can lead to an additional stress in the wafer stack that interferes with the MEMS function, which is tested by an electrical test. This stress can cause individual elements to stick together or to be bent. In the case of a failed test, the problem cannot be located on the MEMS structures because they are opaque to visible light. The defects are to be detected by a topography measurement. This publication discusses the challenge of topography measurement using an low-coherence interference microscope, which is necessary to achieve a high lateral and axial resolution. It turns out that dispersion does not have such a large influence on the measurement signal as the uneven thickness of the cap. The influence of thickness variation could be reduced by measuring both cap surfaces and subtracting them from the MEMS topography. A comparative measurement of the MEMS without a cap shows that the residual deformation is of a similar magnitude. Removing the cap, however, will cause any stress that may be present to dissolve.