Q11Micro and Smart System Technology

Micro-machined Transducers

A micro-machined transducer is a MEMS device that converts a physical, mechanical, thermal, chemical, or optical input signal into an electrical output signal (sensor mode) or vice versa (actuator mode), fabricated using bulk or surface micromachining techniques on a silicon substrate. A representative example is a micro-machined piezoresistive pressure transducer, consisting of a thin silicon diaphragm formed by selectively etching a cavity beneath it, with piezoresistive elements diffused or implanted near the diaphragm edges where mechanical stress is maximum under applied pressure.

When external pressure is applied to the diaphragm, it deflects, producing a stress distribution that is tensile near the edge (where the piezoresistors are placed) and compressive near the center; this stress changes the resistance of the piezoresistive elements (typically arranged as a Wheatstone bridge, two resistors increasing and two decreasing in resistance for maximum bridge sensitivity), producing a differential output voltage proportional to the applied pressure.

The fabrication sequence for such a device begins with a silicon wafer that undergoes selective diffusion or ion implantation to create the piezoresistive elements, followed by bulk micromachining (anisotropic wet etching using KOH or TMAH, exploiting silicon's crystal-plane-dependent etch rates) from the wafer backside to thin the diaphragm region to a controlled thickness of typically a few to tens of micrometers, leaving a thicker, rigid supporting rim around the diaphragm's edge. Metallization and wire-bonding pads are then formed on the front side to allow external electrical connection to the piezoresistive bridge network.

Other common micro-machined transducer types include capacitive transducers (in which a flexible micromachined membrane forms one plate of a variable capacitor, sensing displacement through capacitance change - used in accelerometers and microphones), thermal transducers (exploiting resistive or thermoelectric temperature-dependent effects in suspended, thermally isolated microstructures), and resonant transducers (in which a micromachined resonant beam's natural frequency shifts in response to the measured quantity, such as mass loading or applied force, giving a highly sensitive frequency-domain output signal that is comparatively immune to amplitude-based noise and drift). The common thread across all these types is the exploitation of micromachining to create extremely small, low-mass mechanical structures whose mechanical response to a physical stimulus can be efficiently transduced into a measurable electrical signal, at a scale and cost impossible with conventional macroscopic transducer construction.