## 1 Introduction
The test methods, which are nondestructive and performed at the wafer level, using ordinary wafer-probe test equipment, are much more preferred by foundries. Measurements utilizing electrical input and output are well adapted to this purpose. In addition, electrically-probed test structures offer simple operation, fast response, good repeatability and ease of integration, potentially giving rise to many conceivable applications. Geometrical parameters extracted by such structures and methods could be directly used in the process line for quality control and provide important information for MEMS designers to design, optimize and predict the performance of devices. Works on electrical measurements for geometrical parameters have been reported by using resonance frequency approach [20]. This technique is based on the fusion of analytically calculated and measured resonance frequencies, and a nonlinear least-squares curve fitting analysis is applied to yield geometrical parameters. Laterally resonant comb-drive test structures with prescribed changes in spring width are adopted in this approach. The vibration of the test structure is excited by sweeping the ac drive frequency while resonance is detected by capturing the maximum blur envelope of the test structure under an optical microscope. In order to get more Eigenmodes and hence acquire more reliable data, some researches use Laser Doppler Vibrometry (LDV) instead of the electrical measurements technique to obtain resonance frequencies [18; 21]. The test structures based on micro mirrors are used in the LDV method [21]. The change of thickness of the test structureswill significantly influence the resonant frequencies of the structures. Another test structure is a spring-mass damping structure [18]. The vibration of the test structure is excited by a sonde on one paddle, and the vibration is measured by a LDV while the laser spot is on the other paddle. The springs are twisted in some modes at low frequencies. This structure is used in MEMS fabricated by DRIE. This resonance frequency approach is only suited for the characterization of the geometry of the structural layer. In this paper, an in-line test microstructure for measuring geometrical parameters of surface micromachined thin films is presented. Geometrical parameters of both structural and sacrificial layers, e.g., the thickness and the sidewall etch angle of the PSG layer and the thickness of the polysilicon layer, are obtained by the proposed test structure. The measurement method makes use of the additional resistance of polysilicon thin films caused by step structures. A theoretical model for extracting geometrical parameters based on resistance analysis of step structures is developed. The resistance analysis is confirmed by the finite element simulation. Four-point probe technique is adopted in this method and only current-voltage measurements are required in experiments, which is simple, efficient, nondestructive and compatible with standard IC wafer-level probing techniques. The test structure can be fabricated using surface micromachining process and in situ along with active devices on the same chip. We compare our results to those obtained using a SEM. The results are in agreement.