Measuring 3D geometries of microstructures with the laser-scanning confocal vibrometer microscope

A new type of microscope: the Laser-Scanning Confocal Vibrometer Microscope has been proven to be an ideal tool for out-of-plane vibration measurements in microsystems. This system measures vibrations with heterodyne laser-Doppler technique. The phase demodulation of the carrier of the heterodyne-interferometer detector signal reveals the instantaneous displacement signal. In addition to this well-known property of the heterodyne detector signal, the power of the carrier is proportional to the instantaneous light intensity. We show in this paper that this intensity measurement can be used for an auto-focus control with the vibrometer-laser beam when the microscope objective is moved precisely with a piezoelectric z-positioning stage. The deflection of the z-positioning stage is measured at the maximum signal strength and corresponds to the height information. We demonstrate that the geometry data obtained with the auto-focus routine implemented in our laser-scanning confocal vibrometer microscope matches automatically to the measurement points of the vibration measurements. Our measurements demonstrate that the full-width-half-maximum (FWHM) diameter of the depth response is less than 1 &mgr;m. This enables height measurements with resolutions of a few ten nanometers. Our demonstration system can measure up to 1.5 points in 1 second if the full z-range of 250 &mgr;m is examined.