Improving response of a MEMS capacitive microphone filtering shock noise Armin Saeedi Vahdat a , Ghader Rezazadeh a,n , Saeid Afrang b a Mechanical Engineering Department, Urmia University, Urmia, Iran b Electrical Engineering Department, Urmia University, Urmia, Iran article info Article history: Received 21 December 2010 Received in revised form 4 March 2011 Accepted 9 March 2011 Keywords: Capacitive microphone MEMS Mechanical shock Pull-in instability Frequency response abstract This paper deals with the effects of mechanical shock loads on the stability and dynamic response of a MEMS circular capacitive microphone. As results demonstrate, mechanical shock loads affect the dynamic response and the stability region of the capacitive microphone. The results show that the mechanical shock loads can induce considerable noise in the response of the microphone. Therefore, noise filtering is an important issue to eliminate the output response distortions. To achieve this aim we propose a structure for the capacitive microphone with an electrical circuit in order to eliminate the shock noise. In addition the effect of a delay in shock application is also studied, and it is illustrated that a delay in shock application plays an important role in the stability of the capacitive microphone. & 2011 Elsevier Ltd. All rights reserved. 1. Introduction A microphone is a transducer that converts acoustic energy into electrical energy. The microphones are widely employed in many applications such as cell phones, personal computers, car navigation systems, home-use robots, voice communications devices, hearing-aids, surveillance and military aims, and noise and vibration control [1–3]. Generally microphones can be divided into three types: dynamic microphones, condenser micro- phones and optical microphones [1]. A condenser or capacitive microphone couples acoustical, mechanical and electrical domains. To date because of high sensitivity and low noise levels capacitive microphones are the most common type of silicon microphone [2]. A condenser microphone conceptually consists of a thin vibrating diaphragm and a rigid ground plate that are separated by a small air gap. The relative movement of the diaphragm to the ground plate is solely due to the applied acoustic pressure on the diaphragm, so that the ground plate has no movement as the reference. It means that the ground plate should be thick enough not to move [4]. Capacitive microphones usually need a dc biasing voltage to obtain an ac output if the sensing capacitance changes due to the sound pressure [5]. Traditional microphones, such as Br ¨ uel and Kjær condenser microphones, offer excellent performance, but are costly and currently not suitable for miniaturization [6]. Microelectrome- chanical systems (MEMS) technology has been rapidly growing since its beginning in the 1980s. With the development of MEMS technology, hundreds or thousands of devices can be fabricated together on a single silicon wafer. This leads to the production of MEMS microphones that can approach the performance of tradi- tional microphones with lower cost and smaller size [6]. Currently, commercialization of MEMS devices is a major focus for engineers. One of the most critical issues affecting the commercialization of MEMS devices such as MEMS microphones is their reliability under mechanical shock and impact. MEMS capacitive microphones can be exposed to shock during fabrica- tion, deployment, and operation. Since mechanical shock can induce considerable dynamic loads, it can cause some problems such as chipping, cracking and fracture in the structures [7]. Severe motions and dropping on hard surfaces can induce highly dynamic loads in the MEMS capacitive microphones, which may lead to mechanical and/or electrical failure. Mechanical shock loads can cause capacitive microphones diaphragm to hit the stationary ground plate underneath it, causing stiction [8] and short circuit problems [9] and hence it’s failure. Since the majority of microstructures are fabricated of silicon or polysilicon, they are very tough against bending stresses induced from shock accel- eration. Therefore, failure through stiction and electric short circuits due to contacts between movable and stationary electro- des is more possible than the failure through direct breaking due to contact stresses [7]. A shock can be defined as a force applied suddenly over a short period of time relative to the natural period of the structure [10]. A shock pulse can be characterized by its maximum value, Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/mejo Microelectronics Journal 0026-2692/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2011.03.007 n Corresponding author. E-mail addresses: g.rezazadeh@urmia.ac.ir, g.rezazadeh@mail.urmia.ac.ir (G. Rezazadeh). Microelectronics Journal 42 (2011) 614–621