Strain Measuring Transducer
Summary A strain-sensing transducer designed for detecting small forces in micro-electromechanical systems (MEMS) and nano-electromechanical systems (NEMS) has been reduced to practice and awarded a patent. Because it is fabricated with piezoelectric semiconducting materials, this transducer achieves higher speed and sensitivity than commercial transducers, which are typically made from piezoresistive materials.
The transducer is built with a field-effect transistor (FET) made from a GaAs/AlGaAs heterostructure containing a near surface two dimensional electron gas. The FET is etched into the base of a cantilever arm. A variable physical force applied to the cantilever subjects the FET to a variable strain, creating a piezoelectric effect. This effect results in an electrical response to the applied strain.
Applications Most commercially available strain measuring transducers are silicon-based. Silicon-based sensors require roughly one milliwatt of power for operation, which limits the size of the cantilever and therefore its potential frequency and imaging speed. Cantilevers fabricated with GaAs FETs have very low power dissipation (10 ÂµW), allowing the cantilever to be scaled to extremely small dimensions (3 x 2 x 0.129 Âµm3). These dimensions enable the cantilever to operate at high resonant frequencies (11 MHz), thereby increasing its capacity to scan at high speeds.
Many commercial strain measuring transducers also use displacement sensors which operate separately from the cantilever, limiting sensitivity as size is reduced. Because the GaAs FET is incorporated directly into the cantilever structure, it is referred to as â€œself-sensingâ€. This type of sensor gains force sensitivity as the size of the structure is reduced.
Westervelt, Robert M.
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