Test-structures for wafer level microsystems characterization

Abstract:
The increasing complexity as well as the progressive miniaturization of micro systems call for the development of suitable characterization techniques on wafer level. Micro mechanical sensors and actuators like acceleration sensors, vibration sensors, gyroscopes and micro mirrors form an important group of MEMS that are described as spring-mass-damping (SMD) systems. High throughput manufacturing of these complex microsystems requires the implementation of advanced inline measurement techniques, to quantify characteristic parameters and related scattering caused by the process conditions. In contrast to the ideal rectangular form the MEMS structure is wedge-shaped. Also the mask undercut has to be taken into account. The deviation of geometrical parameters affects the system properties and has to be characterized. Due to the small gaps and the high aspect ratios of microsystem elements (beams, electrodes) optical characterization methods cannot be used. It is also known that built-in mechanical stress influences the behaviour of micro mechanical structures. Raman spectroscopy and X-ray diffraction are suitable for mechanical stress measurements, with a resolution of about 10 MPa. But even stress less than 10 MPa can lead to strong changes in the behaviour of MEMS. The implementation of these measurement systems in the manufacturing process is hardly possible. Furthermore, it is time consuming because the structure has to be scanned. As mentioned above, most important parameters are not accessible by direct measurement techniques. Hence, indirect methods have to be developed in order to evaluate geometrical parameters and mechanical stress.