A High Sensitivity Polysilicon Diaphragm Condenser Microphone

This paper presents the analysis, design, fabrication, and testing of a condenser microphone utilizing a thin low-stress polycrystalline silicon diaphragm suspended above a p+ perforated back plate. The microphone is fabricated using a combination of surface and bulk micromachining techniques in a single wafer process without the need of wafer bonding. The device shows sensitivities of -34 dB (ref. to 1 ) for 2 mm diaphragms with bias of 13 V and -37 dB for 2.6 mm-wide diaphragms at 10 V in good agreement with expected performance calculations. INTRODUCTION Many types of small-sized microphones can be constructed using silicon micromachining techniques at low cost; therefore these devices are promising for consumer electronics. Three types of silicon microphones have been developed: piezoelectric, piezoresistive, and capacitive-type [1]. Capacitive microphones show the highest sensitivity while maintaining a low power consumption. Diaphragms can be made of metal [2], p+ doped silicon [3,4], silicon nitride [5], polyimide and metal [6], and TFE [7]. The most successful devices use silicon as the diaphragm material because of its low intrinsic stress. This stress is very important because it determines the diaphragm sensitivity and its resistance to warpage. These silicon devices use a bulk micromachined p+ diaphragm with a bonded or electroplated stationary electrode. In this paper we use low-stress polysilicon as the diaphragm electrode and a p+ etch-stop silicon plate as the back plate electrode as shown in Fig. 1. The device consists of an n-type silicon substrate, a phosphorus doped polysilicon diaphragm, a p+ perforated back plate, and the metal contacts. This arrangement permits the use of thinner diaphragms with reasonably low stress and does not require any bonding techniques. In the sections below an electrical analog circuit is constructed to determine the microphone sensitivity. Optimal diaphragm edge width, thickness, and air gap are next determined for maximum sensitivity subject to pull-in voltage and processing constraints. Figure 2 shows a top view of a polysilicon diaphragm microphone with 2 mm diaphragm. metal p+ backvent n+ polysilicon diaphragm silicon nitride silicon oxide air gap p+ electrode Figure 1: Cross section of the polysilicon diaphragm condenser microphone polysilicon polysilicon diaphragm