Australian Journal of Basic and Applied Sciences, 3(3): 2847-2855, 2009 ISSN 1991-8178 © 2009, INSInet Publication Corresponding Author: Saeed Rastegar, Department of Electrical Engineering and Computer Science, Babol University of Technology P.O. 484, Babol, Iran. 2847 Application of He'variational Iteration Method to the Estimation of Diaphragm Deflection in Mems Capacitive Microphone Saeed Rastegar, Yashar Toopchi, Bagher Ganji , Hamid Banaei And Mohammad Yosefi Department of Electrical Engineering and Computer Science, Babol University of Technology P.O. 484, Babol, Iran. Abstract: In this paper, He's variational iteration method (VIM) is used to analyze the deflection of polysilicon diaphragm of Micro Electro Mechanical Systems (MEMS) capacitive microphone. The residual stresses in the material used to make the diaphragm change the vibrational characteristics of the microphone diaphragm and consequently influence the microphone’s first resonant frequency, cutoff frequency and sensitivity.The most successful devices use polysilicon as a diaphragm material, because of its residual stress is controllable by high temperature annealing after ion implantation by boron or phosphorous. External acoustic force causes to deflect the diaphragm of the structure and VIM is a powerful analytical method to predict the structural behavior and the microphone performance. Comparison of this new method with the previous approximate solution [1], is applied to assure us about the accuracy of solution. Key words: Diaphragm Deflection, variation iteration method, capacitive Microphone. INTRODUCTION Microphones are transducers that convert acoustic energy into electrical energy. The microphones are widely used in voice communications, hearing aids, noise, and vibration control (Ma and Man, 2002). The silicon microphones have been based on the piezoelectric, piezoresistive and capacitive principles Ying, (1998). The capacitive microphones have been studied by many researchers because of their superior performances, e.g. high sensitivities, low power consumption, flat frequency responses in wide bandwidth, low noise level, stability and reliability Hsu et al., 1998. There are mainly two branches in capacitive microphones - condenser microphone and electret microphones. Electret microphones consist of an electret material, which can store a permanent charge, eliminating the need for external DC biasing. Disadvantages of most electret microphones are the poor retention of electret charges, impossible to electrically regenerate any charges lost after their initial storage on the capacitor and incompatibility with IC process (Zou and Tan, 1998). Condenser microphones consist of a variable gap capacitor. To operate, such microphones need to be biased with a DC voltage (to form a surface charge) (Pappalardo and Caronti, 2002a; 2002b). The condenser microphone has demonstrated the highest achievable sensitivity and very low noise level.Different types of materials can be used for the fabrication of a diaphragm: silicon nitride, annealed polysilicon after ion implantation, polyamide, monocrystaline silicon, and so forth. The residual stresses in these films range from 110 MPa for silicon nitride thin films deposited by the plasma enhanced chemical vapor deposition (PECVD) method to 20 MPa for high temperature annealed polysilicon thin film deposited by the low pressure chemical vapor deposition method (LPCVD) and ion implanted with phosphorous Chowdhury, (2002a). For the maximal acoustical sensitivity it is important to use low stress material as a bending membrane (diaphragm). In a micromechanical component the area is inevitably small, one or two square millimeters. With the use of low-stress material, the bending membrane can be made very flexible Rusanen, (1998). Polysilicon is an alternative thin film for microphone diaphragm, which can be doped to make it electrically conductive. In general, as-deposited LPCVD poly silicon thin films on silicon wafers show large residual stress (about 100 MPa), which makes less interesting for microphones for which high mechanical sensitivity is required. High-temperature annealing of a low-pressure chemical vapor deposition (LPCVD) of poly silicon thin film that is ion implanted with phosphorous can confine the residual stress to as low as 20 MPa. Thus, low-stress poly silicon is a common choice for the diaphragm material Chowdhury, (2002b). The high-sensitivity microphone for the proposed acoustical sensor is the capacitive type and can be fabricated as a single structure using MEMS technology. The microphone