Although a variety of microfabricated accelerators are currently in the research stage of development, most contain surface-micromachined proof masses with piezoresistive, piezoelectric, or capacitive transducers. This instrumentation provides the size scale necessary for accelerometer applications such as navigation, seismology, and acoustic sensing, but can lack the micro-g resolution often required.
Now researchers at the Micro Technologies and Media Laboratories at Massachusetts Institute of Technology (MIT; Cambridge, MA), working with colleagues at Stanford University (Stanford, CA), may have solved the problem with a device that combines a bulk-machined proof mass and support substrate with an optical interferometer. The combination of the microfabricated structure and simple optical detection scheme make small-volume packaging possible.1 In addition, the estimated minimum detectable acceleration of the device is six orders of magnitude below the acceleration of gravityresolution comparable to that reported with surface-micromachined tunneling accelerometers fabricated by a single-mask process.
According to Scott Manalis at the Media Laboratory, a key design element of the interferometer is an interdigital sensing technique originally developed for use with the atomic-force microscope. The accelerometer uses a flexible cantilever to attach the proof mass, which contains a set of extending fingers, to the support structure, with the interleaved fingers linked alternately to both the mass and structure to form an optical diffraction grating. With this design, displacement of the proof mass relative to the support substrate is measured with a standard laser diode and photodetector. Because both optically reflective surfaces of the interferometer are integrated onto the same device, positioning of the diode and photodetector is not critical.
The researchers built the accelerator with a two-mask process on a 500-µm-thick double-side polished silicon wafer using a deep reactive ion etch to define the thickness of the interdigital fingers and the cantilever support. They mounted the frontside pattern, which was covered in a thick layer of photoresist, to a quartz carrier wafer and etched the wafer again with the same technique to release the proof mass from the backside. The accelerometer was then separated from the carrier wafer.
With the interferometric accelerometer, an incident coherent beam diffracts off the reflective surfaces of interleaved fingers (size exaggerated in figure), with the intensities of the diffracted orders determined by the displacement of the proof mass relative to the support frame.
Testing indicates the interferometric accelerometer can detect accelerations on the order of 2 µg/Hz1/2 at 650 Hz. The scientists project that the thermal noise source of the device is somewhere near 90 ng/Hz1/2, so it will be limited by detected and environmental noise sources. In addition to the device size and resolution, they believe another benefit of the design is the possibility that external feedback circuitry may not be necessary (the interdigital sensor is linear over displacements on the order of 100 Å).
According to the researchers, work is still necessary to fine-tune the device fabrication process. For example, they are exploring improvements that include adding a buried etch stop and eliminating the carrier wafer. Ultimately, they "envision a packaging scheme where the illumination source and photodetector can be fabricated as a unit or surface mounted on a circuit board in such a way that they can be aligned within close proximity to the mechanical accelerometer, allowing packaging within a 10-cm3 volume."
Paula Noaker Powell
REFERENCE
- E. B. Cooper et al., App. Phys. Lett. 76, 3316 (May 29, 2000).