How it works
MEMS fabrication uses two main processes: bulk micromachining etches deep into the silicon wafer using KOH or DRIE (Deep Reactive Ion Etching) to create membranes and proof masses; surface micromachining deposits and selectively etches thin films on top of the wafer to build suspended cantilevers and comb-drive structures. The ADXL345 uses polysilicon surface micromachining with a comb-drive differential capacitor. Sensitivity of a capacitive MEMS accelerometer: ΔC = ε₀A·Δx/d², where d is the gap and Δx is proof mass displacement. Piezoresistive MEMS sensors (MPX4115 pressure sensor) use doped polysilicon resistors on a thin silicon membrane; the Wheatstone bridge formed by four resistors converts membrane stress into a millivolt output.
Key points to remember
MEMS devices operate at mechanical resonant frequencies typically in the range 1–100 kHz, far above the signal frequencies of interest (0–1 kHz for inertial sensing), ensuring flat response in the measurement band. The Brownian noise floor of a MEMS accelerometer is a = √(4kBTbω₀/m) μg/√Hz, where b is the damping coefficient — this sets the minimum detectable acceleration. Stiction — the tendency of released microstructures to stick to the substrate — is controlled by using anti-stiction coatings or momentary release HF vapour etch. MEMS gyroscopes (MPU-6050 contains both accelerometer and gyroscope) use Coriolis force on a vibrating ring or tuning fork to sense angular rate; they do not detect gravity and must not be confused with accelerometers in exam answers.
Exam tip
The examiner always asks you to distinguish between bulk and surface micromachining and give one application of each — state that bulk micromachining creates high-aspect-ratio structures for pressure sensors while surface micromachining builds thin-film resonators and comb drives for accelerometers.