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Boyden, E. S., El Rifai, O., Hubert, B., Karpman, M., Roberts, D., A High-Performance Tunneling Accelerometer.
A High-Performance Tunneling Accelerometer (1999)Award-winning class project for MEMS (6.777), Spring 1999. Movie:
AbstractThe goal of this project was to design an accelerometer based on the operating principles of Scanning Probe Microscopy (SPM). The process was constrained to form the tip on the same wafer as the proof mass. The performance targets for the device were as follows:
Full range 5g
Response time 10ms
Cross-axis sensitivity <1%
Self-test output 2.5g
Shock survivability 100ms, 100g shock with 1ms risetime
A number of techniques have been developed which allow surfaces to be imaged with subangstrom resolution. These techniques all involve a tip on the end of a cantilever which is brought into close enough proximity to a surface to interact with the surface. Measurement of the interaction allows determination of the height of the surface at each point. The tip is scanned over the surface to create an image of the surface. These techniques are collectively known as Scanning Probe Microscopy (SPM) modes (How93). The sensitivity of SPM modes to deflection of the cantilever makes them excellent candidates for the fabrication of high sensitivity accelerometers.
The first decision the team faced was selecting the SPM mode for this project. The modes can be grouped by the method used to determine the vertical position of the tip. Three detection methods are in use; piezoresistance, optical and tunneling current.
Position detection using piezoresistance is accomplished by fabricating a piezoresistor at the root of the cantilever and measuring the change in resistance as the cantilever is deflected. This method is relatively poor at measuring small deflections and therefore yields relatively low resolution. Moreover, piezoresistive elements generally have complex variations with temperature. For these reasons, piezoresistive techniques are not appropriate for construction of a high sensitivity accelerometer.
The second, and most common, method of determining the vertical position of the tip is optical. A laser beam is bounced off the back of the cantilever onto a position sensitive photodetector. Deflection of the cantilever causes the position of the beam on the detector to shift. The ratio of the distance from the cantilever to the detector to the length of the cantilever amplifies the motion of the beam. As a result, this method can achieve sub-angstrom resolution.
The optical method has several advantages; it is extremely sensitive, it does not require a vacuum, it can be used with conductive and insulating samples, and it can be used with a variety of SPM modes. However, the need for a laser diode and a photodetector, and the size penalty imposed by the optical method make it undesirable for an SPM accelerometer.
The third method used to determine the vertical position of the tip is to measure the tunneling current between the tip and the substrate. This was the first SPM mode developed. Because the tunneling current is exponentially dependent on the separation between the tip the substrate, the distance between the tip the substrate can be measured to within 0.01 Angstrom. Despite the sensitivity of tunneling, it has been largely displaced by optical methods because it requires a vacuum for imaging oxide forming samples and can only be used with conductive tips and samples. These drawbacks are not serious impediments to the construction of an accelerometer. As a consequence, all of the SPM accelerometer work the team encountered in the literature relies on tunneling current as the detection method.
Based on the above logic and the existence proof of tunneling accelerometers in the literature, the team selected tunneling as the SPM mode for this project.
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