Measuring Lateral Vibrations within all-Silicon Loudspeakers

This application note demonstrates how SmarAct’s PICOSCALE Vibrometer enables the characterization of lateral vibrations in encapsulated all-silicon MEMS loudspeakers. By combining infrared confocal microscopy with non-interferometric measurement modes—such as knife-edge and stroboscopic sampling—lateral displacements down to 20 nm can be visualized and quantified. This approach overcomes the limitations of conventional vibrometry for in-plane motion in silicon-packaged MEMS devices.

Problem

All-silicon MEMS loudspeakers, developed as an alternative to traditional coil-based designs, produce lateral (in-plane) vibrations enclosed in a silicon package. This makes them inaccessible to standard vibrometry tools, which are optimized for out-of-plane motion and transparent surfaces.

Solution

SmarAct’s PICOSCALE Vibrometer, using a 1550 nm infrared laser, enables confocal imaging through silicon layers. It offers specialized non-interferometric measurement modes to detect and analyze lateral vibrations with high spatial and temporal resolution.

Implementation

• Knife-edge Point Measurement: Laser is placed at the actuator edge; lateral motion modulates the reflected light, revealing vibration frequencies and resonances.

• Knife-edge Imaging: Scans the actuator to create spatial maps of amplitude and phase using a digital lock-in amplifier.

• Stroboscopic Sampling: Captures sequences of images synchronized with the excitation signal. A motion-tracking algorithm then quantifies displacements down to ~20 nm.

Results

• Resonance peaks were clearly observed around 60 kHz.

• Knife-edge imaging showed detailed motion patterns of driven actuators.

• Stroboscopic imaging confirmed harmonic lateral motion and provided quantitative displacement data.

Together, these methods enable accurate lateral vibration analysis of encapsulated MEMS devices, complementing out-of-plane interferometry and overcoming conventional measurement limitations.