ACCELERATION

An Extremely Low-Noise
Micromachined Accelerometer
with Custom ASIC Circuitry

The development of a seismic sensor for oil and gas exploration entailed the highly integrated design, simulation, and fabrication of a vacuum-packaged micromachined silicon accelerometer, and a custom mixed-signal ASIC.

Howard Goldberg, Jeff Gannon, and James Marsh,
Applied MEMS Inc., an Input/Output Co.

In seismic exploration for gas and oil, thousands of very sensitive detectors are broadly distributed over several square miles of the Earth’s surface. An energy source such as a vibrator truck or dynamite imparts energy into the earth, creating an elastic wave that is reflected off various layers of rock. By measuring the arrival times of the reflections, seismic sensors can construct an image of subsurface characteristics.

Moving-coil inductive geophones are currently used as seismic sensors. Fifty years of design and performance evolution have led to geophones that are small, rugged, and highly sensitive to motion, and that produce minimal background noise. Designing and building an accelerometer capable of outperforming a geophone presented significant technical challenges, but the effort was justified by direct digital output at the sensor, superior low-frequency response, and other benefits.

Figure 1. An enlarged cross-sectional view of the bulk micromachined accelerometer reveals the variable capacitors between the mass structure (center) and the top and bottom layers.

The Accelerometer
The accelerometer unit’s two principal components are a bulk micromachined capacitive accelerometer and a custom mixed-signal, closed-loop, force-feedback ASIC.

The accelerometer incorporates a moving inertial mass, or proof mass, suspended by springs from a surrounding frame structure. Metal

Figure 2. Four wafers are first etched to form component structures and then bonded together and assembled into a die.

deposited on the upper and lower surfaces of the proof mass creates conductive surfaces (see Figure 1). The upper and lower wafer caps also have deposited metal that creates a variable capacitance between the proof mass and the cap wafers. The assembly is made of four individual silicon wafers (see Figure 2), each etched to form component structures and then collectively bonded to yield the final die assembly (see Photo 1).

To minimize noise, the sensor die is packaged in a high vacuum to produce an evacuated internal cavity containing the proof mass. The

Photo 1. The complete die is constructed from four double-sided wafers (lower left). The middle two wafers constitute the proof mass and springs (upper center and right).

vacuum reduces fluid damping of the proof mass and mitigates noise caused by Brownian motion of internal gas. Considerable engineering effort was necessary to devise a spring design that could achieve a desirable resonant frequency (nominally 1000 Hz) while avoiding undesirable higher order vibrational modes that contribute to noise and stability problems. The accelerometĂr die can function as a stand-alone capacitive accelerometer, but satisfying the performance requirements for its use as a seismic sensor required the development of a custom mixed-signal ASIC.

Photo 2. A mixed-signal ASIC with The <40,000 transistors couples to the accelerometer die. This force feedback, 24-bit, fifth order SD converter achieves very low noise levels.

The ASIC (see Photo 2), which contains ~40,000 integrated transistors, serves several important functions. First, the accelerometer is operated in a closed-loop, force feedback mode. As changes in capacitance are sensed by the ASIC, a restoring electrostŔtic force is applied to maintain the proof mass in a central (neutral) position. Second, the acceleration response, as measured by the feedback force, is digitized by an internal fifth order SD A/D converter. The accelerometer output is an oversampled (128 kHz) digital bitstream.

Commercial third-party modeling and analysis tools were adequate for component design and parametric systems design screening. Optimizing the system’s performance, however, necessitated the development of custom simulation tools that allowed the developers to understand higher order component response effects on overall system performance.

Accelerometer Performance
To validate its performance, the accelerometer has undergone extensive laboratory and field testing. Ambient sensor noise, dynamic range, harmonic distortion, and cross-axis rejection are all important performance characteristics for seismic applications.

The ambient sensor noise as recorded in an isolated quiet chamber is plotted in Figure 3.

Figure 3. The <30 ng/Hz noise floor of the accelerometer is measured in a seismic isolation room. Rotating machinery, which cannot be completely isolated, provides signals at 18, 59, and 78 Hz.

The sensor maintains a noise floor of less than –150 dBg/Hz (<30^–9 gHz ) throughout the seismic frequency range, nominally 3–200 Hz. Rotating machinery operating near the quiet chamber caused the two spikes seen in the plot. The maximum sensor input (before A/D saturation) is nominally 0.2 g peak, providing a dynamic range >115 dB.

Figure 4 shows the frequency response to a 13 Hz sine wave input signal.

Figure 4. The distortion of the accelerometer is compared to a geophone that is normally used for the design application. The distortion (limited by the shaker table) is predicted by simulation to exceed 0.0001% total harmonic distortion.

The total harmonic distortion in the seismic frequency range is <0.02%. Distortion within the vibration table limits the distortion measurements, but analysis predicts a total harmonic distortion of 0.0001%.

A perfect accelerometer will not respond to a vibration input orthogonal to its sensitive axis. Good cross-axis rejection, a measure of normalized accelerometer output to an orthogonal vibration input, is required to accurately resolve the vector components of the vibrational motion. Precision vibration table measurements indicate the accelerometer’s cross-axis rejection to be better than –60 dB.

Although seismic sensors are precision instruments, they must tolerate extreme operating conditions. Field deployments range from arctic tundra to tropical desserts, and interior mountains to coastal swamps. The sensors are transported by helicopters, trucks, ATVs, and boats, and are subjected to very rough handling. Shock and vibration tolerance and operating temperature requirements are accordingly strict. Environmental and field testing have verified the accelerometer’s ability to withstand shocks up to 2500 g (0.5 ms, ¹/₂ sine), and reliably operate over a temperature range of –50 C to 85 C.

Production

Photo 3. The Class 100 clean room facility, one bay of which is shown here, has excess capacity that is available for third-party customers.

Wafer fabrication is performed in a Class 100 clean room (see Photo 3), and accelerometer packaging and test processes in a Class 100,000 clean room. Each accelerometer die is tested before final assembly in the package. The ASIC is fabricated and packaged at a commercial, third-party CMOS foundry and tested in house with a Teradyane A567 advanced mixed-signal IC tester before incorporation in the package.

Commercialization
The finished product, the VectorSeis Module (VSM), incorporates three accelerometers mounted in a geometrically orthogonal configuration. This gives the VSM the capability to resolve the vector components of the elastic wave field propagating through the Earth—the p-waves and s-waves.

The VSM also contains electronics separate from the accelerometer. These perform the essential functions of powering the accelerometer and controlling its operating modes, performing diagnostic tests, decimating and digitally filtering the accelerometer output, synchronizing the VSM modules with the energy source, and remotely relaying the seismic data to recording systems.

Other Applications
Very preliminary tests indicate that the accelerometer could be useful in inertial measurement applications, which also require low sensor noise and good cross-axis rejection. The tests were performed on a vibration table at low-frequency and large-amplitude motion. The output was recorded for 60 s and integrated twice to produce displacement. Overall positional error was <0.1 in. Additional applications may include vehicle electronic suspension control systems, vehicle stability systems, and vehicle rollover detection systems.

Acknowledgments
The authors wish to express their deep gratitude to the many scientists, engineers, technicians, and production professionals who have contributed to this effort, including Bing Fung, Ben Jones, Hai Pham, Tom Poterek, Robert Reid, Larry Rushevsky, Arjun Selvakumar, Philip Simon, Kevin Speller, Marc Stalnaker, Kraig Warren, Duli Yu, and other members of the Wafer Fab Operations and Packing Operations Groups, as well as the rest of the Input/ Output engineering staff.

For further reading on this and related topics, see these Sensors articles. "A Practical Approach to Vibration Detection and Measurement, Parts 1, 2, and 3," February, March, and April 1999 "A 3-Axis Silicon Piezoresistive Accelerometer," February 1999 "Building a Tiny Accelerometer to Detect Very Small Signals," November 2000

Howard Goldberg, Ph.D., is Business and Product Development Manager; Jeff Gannon is Business and Technology Development Manager; and James Marsh is Operations Manager, Applied MEMS Inc., an Input/ Output Co., 1220 Charles E. Selecman Dr., Stafford, TX 77477-2409; 281-552-3040, fax 281-879-2009, jeff_gannon@i-o.com.

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