San Francisco, CA -- To complement his update on the state and future of microelectromechanical systems (MEMS) for automotive and consumer applications, Jiri Marek, senior vice president of Bosch's sensors division (Germany) used his opening slot at ISSCC's Monday plenary session to describe the first integrated inertial sensor modules, combining yaw rate and acceleration sensor stacked on an ASIC in a molded SOIC16w package.
The device is the latest contribution by Bosch's to the automotive industry's Electronics Stability Program. That program was first introduced in 1995 and has increasingly relied on highly integrated MEMS, with up to 90 percent of cars having a MEMS accelerometer and 78 percent having a MEMS-based pressure sensor.
However, as Marek pointed out, these 'first generation' devices were hamstrung by the size of the readout circuit, which prevented the combination of angular-rate and acceleration sensors in a single small-size package. With the SM1540 combined ESP inertial sensor, Bosch has solved that integration problem.
The sensor consists of a combination of two surface micromachined MEMS sensing chips "- one for
angular rate, one for two-axis acceleration "- stacked onto an application specific integrated circuit (ASIC) for readout. The angular-rate sensing element is an electrostatically driven and sensed vibratory gyroscope, which is manufactured in pure silicon surface micromachining, using a slightly modified Bosch
foundry process with an 11-micron-thick polysilicon functional layer to form the movable parts of the micromachined structure.
Bosch's SMI540 is the first ESP inertial sensor to be integrated on an ASIC and package in a molded SOIC16w.
Click on image to enlarge.
The sensor consists of two almost identical masses, connected by a coupling spring to ensure common-mode vibration shapes of the whole structure. Each part has a "drive" frame located on the outer circumference, followed by a "Coriolis" frame and ending at a "detection" frame at the innermost position. All frames of each part are connected via U-shaped springs and the outer and innermost
frames are also anchored via U-shaped springs onto the substrate. The drive frames are excited to a resonant vibration at approximately 15 kHz by electrostatic comb drives operating in an antiparallel direction along the x-axis.
Note, that while the drive motion is translated to the Coriolis frames by the U-shaped springs, the detection frames are hardly affected by the drive motion, leading to a decoupling of drive and detection motion at the detection frames.
The sensors have typical automotive operating temperature ranges from -40 to 120 degree C and tests show noise peaks within approximately +/- 0.1degree/s, a typical output noise density of about 0.004 (°/s) (root)(Hz) and the resolution limit of <3 degree/hr="" corresponds="" to="" amplitude="" changes="" in="" the="" micromechanical="" structure="" of="" as="" small="" as="" an="" atomic="" nucleus.="" "these="" numbers="" can="" only="" be="" achieved="" by="" carefully="" controlling="" tolerances="" during="" fabrication="" processes="" of="" the="" micromechanical="" sensing="" element="" and="" validate="" the="" capability="" and="" maturity="" of="" the="" applied="" design="" concept,"="" said="">3>
Small devices, big future
With such advances, Marek is convinced MEMS have a future that is only beginning to be realized. Already pervasive in automobiles, handsets, gaming, toys and health and fitness applications, Marek sees new applications in autofocus mechanisms for cameras, energy harvesting, micro fuel cells and micromirrors.
However, it is the fusion of different MEMS devices that will truly ignite an explosion in MEMS applications through new capabilities, such as nine degrees of freedom through the combination of 3D accelerometers, 3D gyroscopes and 3D magnetometers. Also, wireless sensors will benefit tremendously from the integrated of RF, energy harvesting and sensing.