MEMS Microphone

Measuring membrane and back-plate vibrations

A MEMS microphone is composed by a diaphragm vibrating between one or two holed back-plates. Measurement of topography and vibration modes of each the diaphragm is challenging :

  • Diaphragm is measured through  the back-plate. High lateral resolution is required
  • The thin 1 um back-plate silicon membrane is transparent for visible and near IR illumination. UV laser source is required in such case

Description

  • Diaphragm diameter : 100 μm
  • Magnification : 20x
  • Lateral resolution : 0.5 μm
  • Excitation frequency : 1 kHz
  • Instrument : DHM R-1000

Vibrations measurement

The amplitude vibration map shows the diaphragm vibrations when the microphone is excited as a speaker by the stroboscopic unit at 1 kHz. The first vibration mode of both diaphragm is represented.

Amplitude vibrations map @ 1kHz

Full field of view Bode plot

Vibration amplitude maps of a microphone back-plate for frequencies between 100 and 500 kHz by step of 4 kHz.

It enables to visualize not only the main resonance modes, but also the spurious and competing modes.

This animation is the generalization of a 1D Bode graph, generally measured on a single point of the sample. It is measured here for the full filed of view.

Membrane and back-plate vibrations

The four measurements here below illustrate the perfect discrimination between the measurement of the vibration of the main membrane and of the back plate.

  • The two first amplitude vibration maps are measured @44 kHz and 104 kHz. Their vibration amplitude is measured inside of the holes of the back-plate. The grid of the back-plate shows nearly no vibrations.
  • The third and fours amplitude vibration maps are measured @362 kHz and 540 kHz. Their vibration amplitude is measured inside of the holes of the back-plate is zero. The vibration of the back-plate is measured on the grid.
Vibration of the membrane of a MEMS microphone measured @ 44 kHz through the holes of the back-plate
Vibration of the membrane of a MEMS microphone measured @ 104 kHz through the holes of the back-plate
Vibration of the back-plate of a MEMS microphone measured @ 362 kHz. One can measure through the holes of the back-plate that the membrane is still.
Vibration of the back-plate of a MEMS microphone measured @ 540 kHz. One can measure through the holes of the back-plate that the membrane is still.