Patent Grant
7239712
The present invention utilizes integrated inductor technology to provide a high sensitivity, linear MEMS microphone.
Micro-Electro-Mechanical Systems (MEMS) involve the integration of mechanical elements, sensors, actuators, and electronics on a common silicon substrate through microfabrication technology. While the electronics of a MEMS device are fabricated using integrated circuit (IC) process sequences, the micromechanical components are fabricated using compatible “micromachining” processes that selectively etch away parts of a silicon wafer or add new structural layers to the wafer to form the mechanical and electromechanical devices.
As discussed by J. Ouellete, The Industrial Physicist, August 1999, the earliest silicon-microphone designs utilized two silicon chips to emulate the advantages of conventional electret microphones. One chip serves as the microphone membrane and the other chip serves as the electrode or backplate. Together, the two chips form a capacitor. As the membrane vibrates in response to sound, the capacitance changes, creating an electrical signal in a circuit connected to the device. Capacitive solutions have the disadvantage of sensitivity, as the capacitance changes as function of 1/d2 where d is the distance between the oscillating membrane and the underlying plate.
Two-chip capacitive silicon microphones provide good acoustical properties, but new manufacturing techniques now enable the fabrication of the entire device on a single chip. Single-chip designs are preferred because they do not require bonding two chips together, but the production process is more complex and expensive.
Piezoresistive and piezoelectric silicon microphones are also utilized. The piezoresistive microphones are single-chip devices that use materials as membranes whose electrical resistivity changes with changes in mechanical stress caused by the deflection of the sound waves. Piezoelectric microphones have a similar design and operation, but the materials of these devices generate differences in electrical potential at the surface instead of changing resistivity. However, piezo systems suffer from both insensitivity and the requirement to utilize expensive pieze materials such as ZnO and AlN.
A MEMS microphone in accordance with the invention utilizes a magnetic mechanism to achieve the same result as capacitive or piezo devices, but with several advantages. The present invention is based upon a more standard integrated inductor technology with the addition of an etched out underlying layer in the silicon to form the microphone cavity. The idea is to suspend an inductor over another fixed inductor such that the magnetic field induced from one induces an electric potential across another.
Optionally, depending upon sensitivity requirements, either or both inductors may be driven with either a DC or AC signal. The induced signal on the recipient inductor, relates to the displacement current induced by the moving B-field. In the case of the DC signal, the signal is induced as a function of distance (Maxwell's 2nd equation). In the case of the AC signal, an extra term (and hence extra sensitivity) associated with induced E-field leads to more output signal. (Maxwell's 3rd equation).
Following the formation of the cavity 210 in the dielectric material, a layer of conductive material 212 is formed over the dielectric material 208 and over the cavity 210. The conductive layer 212 is then patterned to form a vibrating inductor structure L2 that is suspended over the cavity 210 and over and separated from the stationary embedded inductor structure L1, as shown in
As shown in
It should be understood that various alternatives to the embodiments of the invention described herein may be employed in practicing the invention. It is intended that the following claims define the scope of the invention and that structures and methods within the scope of these claims and their equivalents be covered thereby.
| Number | Name | Date | Kind |
|---|---|---|---|
| 7054460 | Rombach et al. | May 2006 | B2 |
