## 1 Introduction  
Surface micromachining process is one of the most popular technologies used to manufacture microelectromechanical systems (MEMS) devices. A typical surface micromachining process involves thin films, normally structural and sacrificial layers, alternately deposited on the substrate and patterned. The MEMS parts are made of the structural layer while the sacrificial layer serves as a stable temporary support that can be removed to free the structural layer on top [1]. Polysilicon and phosphosilicate glass (PSG) are often used as the structural and sacrificial material, respectively. The thickness and the sidewall etch angle of polysilicon and PSG thin films are fundamental parametersfor quality control in a surface fabrication process line. Besides, the correct thickness is vital for guaranteeing the performance of MEMS devices. On the one hand, mechanical responses of membrane/beam-based MEMS devices, such as mechanical resonance frequency, restoring force, longitudinal strain and deflection, etc., usually depend on the thickness of the structural layer. For example, the mechanical sensitivity of MEMS microphones is influenced by film thickness [2]. Thickness of the membrane has strong effect on the maximum deflection of membrane-based pressure sensor [3]. Thickness and other dimensions determine the resonance frequency of lateral comb drive devices [4]. On the other hand, the thickness of the sacrificial layer limits the longitudinal movement of a freestanding structure. The pull-in voltages of double-clamped beams and cantilever beams, for instance, are both related to the gap between the beam and the ground plane, which is defined the thickness of the sacrificial layer [5]. Moreover, in order to extract mechanical and thermal properties of the thin film, including Young's modulus, residual stress, thermal conductivity, heat capacity, etc., the knowledge of the film thickness is often required [5; 6; 7; 8]. Therefore, it is very necessary to characterize the thickness and other geometrical parameters of surface micromachined thin films. There are many available techniques used to measure the thickness and sidewall etch angle of the thin films. Surface profilometer is widely used for the determination of the film thickness and shape information [9]. The measurement method is to prepare thin-film steps and measure them by moving a very sharp stylus tip over the step surface and tracing the stylus at each location. Scanning electron microscope (SEM) is one of effective tools to characterize geometrical parameters of thin films [10]. To obtain the film thickness and sidewall etch angle, the sample is sliced to reveal the cross section of the films and the viewing angle of SEM is aligned to be orthogonal to the sample surface. Another conventional approach to evaluate film thickness is performed by ellipsometers, which are optical instruments that detect the ellipticity of polarized light [11]. The film thickness is extracted by measuring the change in polarization upon reflection or transmission through the film. Various interferometric methods have been developed to determine film thickness [12; 13; 14; 15; 16]. These methods are principally divided into single and multiple beam interferometry and basically exploit the reflectance of surfaces as well as the intensity of light. The film thickness is estimated by analyzing interference pattern formation [17]. The aforementioned techniques for determining geometrical parameters are either optical or mechanical methods, most of which uses specific instruments that are time consuming, bulky and expensive. Often the sample has to be broken or to be prepared by metallographic techniques or the focused ion beam technique [18]. Although useful in laboratory investigations, the operations of such methods are inconvenient for batch-fabricated industrial scale measurements or post-packaged measurements, which are in generally not acceptable for in-line tests [19].