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Semiconductor applications

Vibration measurements through silicon with IR laser-light

Both SmarAct’s PICOSCAN Vibrometer and PICOSCALE Interferometer are based on an infrared 1550 nm laser source. A unique advantage of using infrared light is that it can be used to look through silicon to measure vibrations of underlying structures. This means that micromechanical devices that are enclosed in a silicon housing can also be examined.

Experimental Setup

Here, we demonstrate the use of a focused IR laser beam (1550 nm) to look through a silicon window and image the vibrations of an underlying structure. For the test we used a PICOSCAN Vibrometer which is a combined confocal microscope and raster-scanning vibrometer. The test sample was an AFM microcantilever.

The cantilever vibrations are recorded while a silicon wafer (0.3 mm thick) was placed between the laser source and the sample. The silicon wafer fragment was partly coated with gold. For the experiment the uncoated edge was used. To induce vibrations, the cantilever was mounted on a piezo actuator that was driven at 100 kHz.

Microscopy and vibration images

The 200 µm × 200 µm area was scanned with 200 x 200 pixels. The shown images are unprocessed. The reflection images show the microscopic details of the sample. The amplitude images show the second bending mode of the cantilever. The phase images are not shown.

Although the 0.3 mm thick silicon window reduces the signal intensity, the sample and its vibrations can still be clearly detected. These results show that it is possible to image microscopic structures and their vibrations, even if they are packed in a silicon enclosure. Single point measurements through a silicon window can also be performed with a PICOSCALE Interferometer in combination with a focusing sensor head.