Novel Application of Amorphous Silicon in Electrostatic Loudspeakers
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NOVEL APPLICATION OF AMORPHOUS SILICON IN ELECTROSTATIC LOUDSPEAKERS R.E.I. Schropp, M. Smaits*, H. Meiling, W.G.J.H.M. van Sark, M.M. Boone", J. Bezemer, and W.F. van der Weg Department of Atomic and Interface Physics, Debye Institute, Utrecht University, P.O. Box 80,000, 3508 TA Utrecht, The Netherlands *) Final, Speykstraat 2, 3572 XC Utrecht, The Netherlands *) Faculty of Applied Physics, Section for Seismics and Acoustics, Delft University of Technology, Lorentzweg 1, 2628 CJ Delft, The Netherlands
ABSTRACT A thin film of amorphous silicon (a-Si:H) has been developed for application as the charge retention layer in electrostatic loudspeakers. The vibrating diaphragm in this type of loudspeakers is usually coated with a thin film in order to assure homogeneous charge distribution across the surface. This thin film should be capable of maintaining a high surface charge density and possess a high lateral resistivity to prevent charge displacement. Furthermore, it should be mechanically stable, capable of accomodating mechanical deformation, and resistant against humidity. Conventionally, a graphite layer is used in these applications. However, the conventional layers are frequently unstable and suffer from charge displacement effects eventually leading to electric breakdown. Further requirements are that the film can be deposited continuously and homogeneously over large areas in excess of 1 m 2 and that the deposition technique is compatible with the properties of the thin membrane. We investigated small-area prototypes of electrostatic loudspeakers with a-Si:H thin films deposited on polyimide substrates in the Utrecht deposition system, ASTER. Plasma-deposited amorphous silicon films fabricated under certain conditions are shown to meet all of the above requirements.
INTRODUCTION The capability of hydrogenated amorphous silicon (a-Si:H) to be uniformly deposited over a large area has already been used in numerous applications, such as in solar modules, large area switching arrays, and large area electrophotoreceptors. The application in photoreceptors in particular has inspired us to develop an amorphous silicon thin film which is suitable as the charge retention layer on the vibrating foils in electrostatic loudspeakers. Compared to the more commonly used electrodynamic loudspeakers, this type of loudspeakers has much lower distortion characteristics. In these loudspeakers, one aims at obtaining a large area thin film which can be either positively or negatively charged by applying a high voltage but at the same time can hold large surface charge densities which do not displace laterally. Hydrogenated amorphous silicon thin films can be compositionally controlled to possess these capabilities. Obviously, one does not seek the high photosensitivity as is required in electrophotography. As there are several ways to quench the large photoresponse in a-Si:H, the high resistance requirement for the charge retention layers under room light conditions does not impose a major problem. This paper will focus on metho
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