Interview questions & answers
Q1. What is MEMS and what makes it different from conventional semiconductor devices?
MEMS integrates mechanical structures (beams, membranes, gears) with electrical transduction elements on a silicon chip using photolithographic fabrication processes, unlike conventional ICs that contain only electronic components. The Bosch BMA456 accelerometer contains a 300 μm silicon proof mass suspended by spring beams and differential capacitive sensing electrodes, all fabricated on a single die. The key distinction is that MEMS devices interact with physical quantities (force, pressure, acceleration) through micron-scale mechanical motion, which requires specialized fabrication processes like deep reactive ion etching (DRIE).
Follow-up: What are the main differences between surface micromachining and bulk micromachining?
Q2. Explain the working principle of a MEMS capacitive accelerometer.
A MEMS capacitive accelerometer uses a proof mass suspended by spring beams; when the device accelerates, the proof mass displaces relative to the frame, changing the differential capacitance between movable fingers and fixed electrodes, which is sensed by a charge amplifier. The ADXL345 uses a polysilicon proof mass with 32 differential sensing capacitors, each with a nominal gap of 2 μm, giving a sensitivity of 57 mg/LSB at ±2 g range. Differential capacitance sensing cancels common-mode effects like temperature drift, making it far more accurate than single-ended capacitance measurement.
Follow-up: How does the spring constant of the suspension beams affect the accelerometer's sensitivity and bandwidth?
Q3. What is the difference between surface micromachining and bulk micromachining?
Surface micromachining builds mechanical structures by depositing and selectively etching thin films on top of a silicon substrate, using sacrificial layers (typically SiO₂) to create free-standing structures, while bulk micromachining removes material from the silicon wafer itself to create structures. Texas Instruments' DMD chip uses surface micromachined aluminum mirrors over a sacrificial layer, while Bosch automotive pressure sensors use bulk micromachined silicon diaphragms etched from the back side with KOH. Surface micromachining allows smaller structures and CMOS integration, while bulk micromachining gives thicker, more robust structures for pressure and inertial sensing.
Follow-up: What is a sacrificial layer and why is it needed in surface micromachining?
Q4. How does a MEMS piezoresistive pressure sensor work?
A MEMS piezoresistive pressure sensor uses a thin silicon diaphragm with implanted piezoresistors at the points of maximum stress; when pressure deflects the diaphragm, the resistor values change due to the piezoresistive effect, and the resulting Wheatstone bridge imbalance gives an output voltage proportional to pressure. The Bosch BMP388 barometric sensor uses a 200 μm square silicon diaphragm with four p-type piezoresistors in a full Wheatstone bridge, giving a sensitivity of ~0.016 Pa at sea level. The gauge factor of silicon piezoresistors is approximately 100× higher than metal strain gauges, enabling much smaller diaphragms for the same sensitivity.
Follow-up: Why is a full Wheatstone bridge configuration used rather than a single piezoresistor measurement?
Q5. What is DRIE (Deep Reactive Ion Etching) and why is it important for MEMS fabrication?
DRIE is a plasma etching process using alternating etch (SF₆) and passivation (C₄F₈) cycles (Bosch process) to create deep, high-aspect-ratio trenches with nearly vertical sidewalls in silicon, enabling structures that are tens to hundreds of microns deep with widths of a few microns. The Bosch BMA series accelerometers use DRIE to create 70 μm deep sense fingers with 2 μm gaps, achieving high capacitance per unit area. Without DRIE, wet etching with KOH would produce angled sidewalls limited by crystallographic planes, preventing high-aspect-ratio capacitive sensing structures.
Follow-up: What is the aspect ratio achievable with DRIE compared to wet KOH etching?
Q6. Explain the working principle of a MEMS gyroscope.
A MEMS gyroscope uses the Coriolis effect: a proof mass is driven into continuous oscillation along one axis (drive axis), and when the device rotates, the Coriolis force deflects the mass along the orthogonal sense axis, with deflection proportional to the angular rate. The STMicroelectronics L3GD20H 3-axis gyroscope uses a polysilicon tuning fork structure, with the drive oscillation maintained by electrostatic actuation and the Coriolis deflection sensed capacitively. The scale factor is expressed in mV/(°/s) or LSB/(°/s), and the noise density (typically 0.03°/s/√Hz) limits the minimum detectable rotation rate.
Follow-up: What is quadrature error in a MEMS gyroscope and how is it compensated?
Q7. What materials are commonly used in MEMS fabrication and why?
Silicon is the primary structural material due to its excellent elastic properties (no creep, Young's modulus 130–190 GPa depending on crystal orientation), while polysilicon is used for surface-micromachined structures, silicon dioxide for sacrificial layers and insulation, silicon nitride for membranes and passivation, and aluminum or gold for electrical contacts. Piezoelectric MEMS use PZT (lead zirconate titanate) or AlN (aluminum nitride) for actuation and sensing, as in the Vesper VM1010 MEMS microphone that uses AlN. Single-crystal silicon is preferred for high-Q resonators because its internal damping is much lower than polysilicon.
Follow-up: Why does single-crystal silicon not exhibit fatigue or creep under cyclic mechanical loading?
Q8. How does a MEMS microphone work and what advantages does it have over electret condenser microphones?
A MEMS microphone uses a thin silicon diaphragm (backplate) separated from a rigid perforated backplate by an air gap, forming a capacitor; sound pressure deflects the diaphragm, changing the capacitance, which is converted to voltage by an ASIC. The Knowles SPH0645 MEMS microphone has a signal-to-noise ratio of 65 dBSPL and occupies 3.76 × 2.95 mm, far smaller than a conventional electret mic. MEMS microphones are fabricated using standard semiconductor processes, enabling direct integration with the ASIC and eliminating the manual assembly needed for electret microphones.
Follow-up: What is acoustic overload point (AOP) and why is it important for MEMS microphone selection?
Q9. What is stiction in MEMS and how is it prevented?
Stiction is the permanent adhesion of released MEMS structures to the substrate due to capillary, van der Waals, or electrostatic forces during or after the wet release etch, preventing the structure from moving. Texas Instruments coats DMD mirrors with an anti-stiction self-assembled monolayer (SAM) during fabrication to reduce surface energy and prevent contact adhesion. Supercritical CO₂ drying (critical point drying) is used during the HF release etch of polysilicon accelerometers to avoid the surface tension that causes capillary stiction.
Follow-up: What is the difference between in-use stiction and release stiction in MEMS?
Q10. What is the Q-factor of a MEMS resonator and what limits it?
The Q-factor (quality factor) of a MEMS resonator is Q = f₀/Δf = 2πf₀ × (stored energy / power dissipated), with higher Q giving sharper resonance, lower phase noise, and better frequency selectivity. A quartz tuning fork oscillator (32.768 kHz) has Q ≈ 10,000–100,000, while a silicon MEMS resonator in vacuum achieves Q > 100,000 at the same frequency. The main loss mechanisms are thermoelastic damping (TED), anchor losses, and squeeze-film air damping; vacuum packaging (used in Bosch BMI088 IMU) removes air damping and allows Q values over 10,000.
Follow-up: Why is vacuum packaging used in MEMS gyroscopes but not typically in accelerometers?
Q11. What is the Coriolis effect and how is it exploited in MEMS inertial sensors?
The Coriolis effect is a pseudo-force F = -2m(Ω × v) acting on a mass m moving with velocity v in a rotating reference frame with angular velocity Ω, perpendicular to both the velocity and rotation vectors. In a MEMS gyroscope, the drive velocity v is known from the driven resonance amplitude, so measuring the Coriolis-induced deflection directly gives the rotation rate Ω. InvenSense MPU-6050 uses this principle in a single chip containing both a 3-axis MEMS gyroscope and 3-axis accelerometer, enabling attitude estimation for smartphones and drones.
Follow-up: Why must the drive mode resonant frequency be precisely matched to the sense mode frequency for optimal gyroscope sensitivity?
Q12. How are MEMS pressure sensors used in automotive applications?
MEMS pressure sensors measure manifold absolute pressure (MAP), tire pressure (TPMS), and exhaust backpressure in automotive systems, with silicon piezoresistive or capacitive sensors replacing bulky mechanical gauges. The Bosch BPS300 automotive MAP sensor uses a bulk-micromachined silicon diaphragm rated for -40°C to 150°C and 17 bar, packaged in a media-isolated housing resistant to fuel vapor. Silicon sensors are preferred over ceramic alternatives because they can be batch-fabricated and trimmed to ±0.5% accuracy using laser trimming of the Wheatstone bridge resistors.
Follow-up: What is media isolation in an automotive pressure sensor and why is it necessary?
Q13. What is packaging's role in MEMS devices and what are the unique challenges?
MEMS packaging must protect fragile mechanical structures from external stress, moisture, and particles while allowing the device to access the physical stimulus (pressure, acceleration, sound), unlike IC packaging where hermetic sealing is sufficient. The Bosch BMA456 uses wafer-level capping (WLC) where a silicon cap wafer is bonded over the MEMS die at the wafer level before dicing, maintaining vacuum for the gyroscope while the accelerometer operates at atmospheric pressure. Mechanical stress from package molding can shift the zero-g offset of an accelerometer by several mg, requiring stress-isolation structures or package-level compensation.
Follow-up: What is wafer-level packaging (WLP) and why is it preferred for MEMS over conventional die-level packaging?
Q14. What is an IMU (Inertial Measurement Unit) and what MEMS sensors does it contain?
An IMU is a device that measures and reports specific force, angular rate, and sometimes magnetic field using a combination of accelerometers, gyroscopes, and magnetometers integrated in a single package. The Bosch BMI088 IMU combines a triaxial accelerometer and triaxial gyroscope in a 3 × 4.5 × 0.95 mm LGA package with 6-axis measurement at up to 800 Hz output data rate, designed for vibration-robustness in drones. Combining sensors in one package reduces assembly cost, synchronizes measurement timestamps, and enables sensor fusion algorithms like complementary or Kalman filters for attitude estimation.
Follow-up: What is sensor fusion in an IMU and what algorithm is commonly used to combine gyroscope and accelerometer data?
Q15. What is thermoelastic damping (TED) in MEMS resonators?
Thermoelastic damping is an intrinsic energy loss mechanism in vibrating structures where the stress-induced temperature gradients cause irreversible heat flow, converting mechanical energy to heat through the thermoelastic coupling of silicon. TED dominates the Q-factor of silicon beams at frequencies around 1–100 MHz and its magnitude depends on beam geometry, thermal conductivity, and the Zener frequency (f_Z = κ/(ρCp b²)), where b is the beam thickness. Designing MEMS resonators with aspect ratios where the operating frequency is far from the Zener frequency minimizes TED, a design principle used in SiTime MEMS oscillators.
Follow-up: How does vacuum packaging help if TED is an intrinsic mechanical loss not related to air damping?
Common misconceptions
Misconception: MEMS are just very small mechanical devices made with precision machining.
Correct: MEMS are fabricated using photolithographic processes derived from IC manufacturing, not machining, enabling batch fabrication of millions of devices per wafer with micron-level features.
Misconception: A MEMS accelerometer directly measures velocity.
Correct: A MEMS accelerometer measures specific force (acceleration), and velocity must be obtained by integrating the acceleration signal, which accumulates drift errors over time.
Misconception: Surface micromachining always produces thicker, stronger structures than bulk micromachining.
Correct: Surface micromachining produces thin (1–10 μm) deposited film structures, while bulk micromachining can produce structures hundreds of microns thick by etching the silicon wafer itself.
Misconception: Vacuum packaging is needed for all MEMS sensors to prevent air damping.
Correct: Gyroscopes require vacuum packaging to achieve the high Q needed for low noise, but accelerometers often use air damping deliberately to suppress resonant overshoot and improve step response.