Page 29 - Summer 2008

MANAGING ACOUSTIC FEEDBACK:
MICRO ELECTRO MECHANICAL SYSTEMS (MEMS) CONTACT MICROPHONES FOR MUSICAL INSTRUMENTS
Rob O’Reilly, Alex Khenkin and Kieran Harney
Analog Devices, Micromachined Products Division Cambridge, Massachusetts 02139
Introduction
The integrated circuit wafer fabrica- tion process has matured and scaled rapidly since the invention of the transistor. Semiconductor tech- nology has the major advantage of scale and reproducibility; many thousands of identical chips can be produced on one wafer and the repeatability from wafer to wafer is very well controlled. Silicon integrated circuit technology now has the capability of creating device geome- tries of less than 100 nanometers.
Micro Electro Mechanical Systems
(MEMS) is a technology that builds on the core silicon fabri- cation infrastructure that has been developed for the inte- grated circuit industry. Micromechanical structures are cre- ated on a silicon wafer by etching defined patterns on a sili- con substrate to form core sensor elements or mechanical actuators that can move fractions of a micron. Pressure sen- sors were one of the first high-volume applications and hun- dreds of millions of MEMS pressure sensors are now in use in applications such as pressure-sensing in engine manifolds and tire pressure monitoring systems. MEMS accelerometers have been used in automotive applications for over 15 years as crash sensors for airbag deployment, for rollover detection and car alarm systems. More recently MEMS accelerometers are used for motion sensing in consumer applications such as video games and cell phones. MEMS micro-mirror optical actuators have found use in overhead projectors and projec- tion televisions. In recent years MEMS microphones have begun to proliferate in the broad consumer market including cell phones, Bluetooth headsets, personal computers and dig- ital cameras. In university and industry-based research departments today there is significant investment in bio- MEMS which is focusing on the use of micro- or nano-tech- nologies to execute diverse applications from DNA testing to disposable medical diagnostic kits.
This article summarizes some of the key technologies deployed in MEMS accelerometers and then discusses how this technology may bring a new dimension to acoustic transducers for musical instruments.
MEMS accelerometer technology
The core element of a typical MEMS accelerometer is a moving beam structure. This element is typically comprised of two sets of fingers—one set fixed to a solid ground plane on a substrate and the other set attached to a known mass mounted on springs that can move in response to an applied acceleration. Under an applied acceleration there is a change
“Suppose we use a surface transducer that measures the acceleration of an instrument’s body that is so lightweight it does not affect the instrument that it is measuring.”
in capacitance sensed between the fixed and 1
moving beam fingers. It is the change in capacitance (i.e., the relative change in spacing between the fingers) that is measured and con- verted to acceleration units, “g.” (1 g, the accel- eration due to gravity on Earth, is about 9.8 meters per second per second or about 32 feet per second per second). Figure 1 illustrates the MEMS accelerometer concept while Fig. 2 provides an example of an actual MEMS sen- sor at 1500 magnification
The dimensions of these MEMS struc- tures are in the order of microns, requiring very high precision silicon photolithography
and etching process technologies. MEMS structures are typi- cally formed from single crystal silicon or from polysilicon that is deposited at very high temperatures on the surface of a single crystal silicon wafer. Structures with very different mechanical characteristics can be created using this flexible technology. One mechanical parameter that can be con- trolled and varied is spring stiffness. The mass of the sense element and the damping of the structure can also be modi- fied by design. Sensors can be produced to measure fractions
Fig. 1. MEMS accelerometer structure diagram.
Fig. 2. ±50g MEMS accelerometer physical structure (1500x).
28 Acoustics Today, July 2008