Hans-Jörg Timme

Silicon micromachined microphone chip at Siemens

Applications ranging from hearing aids over communication to noise cancellation open up a high volume market for low‐cost, batch producible and reliable microphones. To obey these conditions, a single‐chip capacitive microphone has been developed at Siemens, utilizing a modified standard CMOS process with adjacent bulk micromachining. In a first step, the microphone is integrated with a source follower, enabling low output impedance of the signal. The technology allows for the future integration of advanced circuitry. The microphone consists of an acoustically sensitive polycrystalline silicon membrane and a highly perforated back‐plate as the counter electrode. To achieve highly sensitive devices, special emphasis was given to the stress of the polycrystalline silicon membrane, which should be slightly tensile. Another key issue during the fabrication and in operation is to prevent stiction of the sensitive membrane. Since the overall chip size is below 3‐mm side length, surface mounting in low‐cost SMD packages is possible.