MEMS and sensors

Learn about MEMS features, capacitive accelerometers, sacrificial-layer etching, the piezoresistive effect and Hall sensors through multiple-choice questions.

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MEMS and sensors: key points

MEMS (micro-electro-mechanical systems) are devices that use semiconductor microfabrication to build tiny mechanical structures together with electronic circuits. They are widely used as accelerometers, gyroscopes, microphones and pressure sensors in smartphones.

Understand sacrificial-layer etching, which frees movable structures, and the principles that turn displacement, stress or magnetic fields into electrical signals (capacitance, the piezoresistive effect and the Hall effect).

All MEMS and sensors questions (50)

  1. Which correctly describes MEMS?
  2. What is the principle by which a capacitive MEMS accelerometer detects acceleration?
  3. What is the purpose of sacrificial-layer etching in MEMS fabrication?
  4. Which correctly describes the piezoresistive effect used in semiconductor pressure sensors?
  5. What does a sensor using the Hall effect (Hall element) mainly detect?
  6. Which force does a vibratory MEMS gyroscope use to detect angular velocity?
  7. Which correctly describes deep RIE (the Bosch process) used in MEMS fabrication?
  8. What is the typical way a MEMS microphone converts sound into an electrical signal?
  9. How does the forward voltage of a silicon PN junction at constant current depend on temperature?
  10. What is the operating principle of the DMD (digital micromirror device) used in projectors?
  11. What is the general ink ejection method of inkjet printer heads made with MEMS technology?
  12. Which phenomenon do BAW filters, used as RF filters in smartphones and similar devices, use?
  13. What is the main reason for sealing the moving parts of MEMS gyroscopes and similar devices in vacuum?
  14. What is the phenomenon called in which tiny moving parts stick to the substrate due to surface tension or similar forces during MEMS fabrication or use, and stop moving?
  15. What is the detection principle of semiconductor gas sensors using tin oxide or similar materials?
  16. What does the sensor used in a smartphone's electronic compass detect?
  17. What can be estimated from changes in air pressure using a smartphone's MEMS pressure sensor?
  18. Why are SOI wafers often used in MEMS fabrication?
  19. What is the method called that bonds silicon and glass by applying voltage while heating, used for MEMS sealing and similar purposes?
  20. Which MEMS device is used to scan laser light in automotive LiDAR and similar systems?
  21. How can the resonant frequency of a MEMS structure consisting of a spring and a mass be raised?
  22. Which MEMS sensor has been used to decide when to deploy car airbags?
  23. When removing an SiO2 sacrificial layer to free a MEMS structure, which method is suitable for preventing stiction?
  24. Which MEMS fabrication method builds movable structures by processing thin films deposited on the substrate?
  25. Which correctly describes how MEMS sensors are combined with their signal-processing circuits?
  26. Why does a MEMS accelerometer detect about 1 G of acceleration even when stationary?
  27. What is the processing called that combines information from accelerometers, gyroscopes, magnetometers and other sensors to determine orientation and motion more accurately?
  28. What is an advantage of MEMS oscillators, sometimes used instead of quartz crystals as clock sources?
  29. Which MEMS sensor measures flow rate using the fact that gas flow carries away heat from a heater?
  30. What is an advantage of RF MEMS switches, which switch RF signals by mechanically opening and closing contacts?
  31. Which temperature sensor uses the property that its electrical resistance changes greatly with temperature?
  32. Which is the most appropriate scale for the movable structures made by MEMS?
  33. What is the principle of the optical sensor that measures heart rate at the wrist in smartwatches and similar devices?
  34. Which sensor does a smartphone use to automatically adjust screen brightness to its surroundings?
  35. What is the general principle of the sensor a smartphone uses to turn off the screen when your face approaches during a call, preventing accidental touches?
  36. What is the driving principle of the "comb-drive" structure used as a MEMS actuator?
  37. Which device uses acoustic waves traveling along the surface of a piezoelectric substrate and serves as a high-frequency filter in smartphones and similar?
  38. Which sensor, widely used in motion sensors that turn on lights when a person approaches, uses a material that generates charge when its temperature changes?
  39. Which is an example of MEMS "energy harvesting"?
  40. Which technology builds tiny channels on a chip to perform chemical analysis or testing with small amounts of liquid?
  41. What is the principle of the NDIR gas sensor often used to measure indoor carbon dioxide (CO2) concentration?
  42. Which MEMS-fabricated ultrasonic elements are sometimes used in under-display fingerprint sensors in smartphones and in medical ultrasound?
  43. In surface micromachining, what problem occurs if a structural thin film such as polysilicon has large internal residual stress?
  44. In a MEMS electrostatic actuator, if the voltage between two parallel electrodes is doubled, by roughly what factor does the electrostatic attraction between them change? (Assume the gap is unchanged.)
  45. When choosing the measurement range of a MEMS accelerometer (for example, ±2 g versus ±16 g), which reasoning is correct?
  46. In a parallel-plate electrostatic actuator, what is the phenomenon called in which, as the voltage rises, the movable electrode suddenly snaps to the fixed electrode once it passes a certain position?
  47. Which method is widely used to evaluate the bias stability (slow drift of the output) of MEMS gyroscopes?
  48. What is the damping called that arises when a MEMS movable plate moves toward a nearby fixed surface and squeezes out the gas between them?
  49. In very small MEMS accelerometers and gyroscopes, which noise, caused by the proof mass jiggling randomly through thermal interaction with surrounding gas molecules and the like, is one limit on resolution?
  50. Which statement correctly describes detecting very small masses by using the change in resonant frequency when material adheres to a tiny cantilever or a quartz crystal resonator?

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