The present innovation, developed at NASA’s Goddard Space Flight Center, comprises several piezoresistor sensor configurations for sensing rotation or torque that are superior to those currently in use in microelectromechanical (MEMS) devices. These may be used for sensing rotation/torque or any other quantity that can be converted by the sensor to a rotation (as is frequently done in sensors), and are therefore of wide applicability in current and future MEMS sensors.
Due to both cost and performance considerations, the use of silicon-based MEMS sensors is widespread and still growing for sensing pressures, accelerations, displacements, forces, stresses, torques, rotation, and chemicals. The piezoresistive properties (resistance change with stress/strain) of silicon are used in many of these sensors. A measurement creates mechanical stresses in the silicon sensor, which are in turn converted to an electrical signal or read using a piezoresistor or combination of piezoresistors on the sensor. These piezoresistors are ideally configured to be selective to the quantity of interest, and also to maximize sensitivity.
A growing number of these sensors, such as those used in micromirrors, aeronautical skin friction sensors, or tactile sensors, are configured so as to require the measurement of rotation of a portion of the silicon sensor, or a torque exerted on it. Typically, the torsional or shear stress created by the rotation/torque is sensed by a piezoresistive configuration. The piezoresistors are usually implanted in the silicon, and the sensing resistor region is isolated by using a diode effect (p-type resistors in n-type silicon background, or vice versa), or some other isolation mechanism. They are orientated in particular, dopant-dependant directions relative to the crystal structure of the silicon for optimal response to the torsional or shear stress. An excitation current then produces a voltage across the output terminals that is proportional to the torsional stress.
The piezoresistors often have to be located on small, thin beams or hinges to produce adequate sensitivity. Concentrating forces on a small area produces a higher stress, and thereby a higher output. The ability to increase sensitivity by decreasing the dimensions of this area is limited either by the resolution of the photolithographic and other process technology that are used to fabricate such sensors, or by other design constraints such as undesirable mechanical resonances or cross-sensitivities. That’s why the ability to improve sensitivity without needing a significant advance in sensor fabrication process technology or design tradeoffs is highly desirable. The Goddard technology accomplishes this with several piezoresistor sensor configurations for sensing rotation or torque that are superior to those currently used in MEMs devices.
While this technology is fabricated on silicon, the arguments and design are also applicable to other similar semiconductor materials.