A commercial scanning imaging white-light interferometer designed for measuring surface profiles of stationary objects has been modified into an interferometric instrument for imaging vibrating microelectromechanical structures. The modified instrument operates in a stroboscopic mode, generating a set of interferograms at a selected instant in the vibrational cycle. A number of sets of interferograms can be acquired at different instants of time corresponding to small increments of phase through the vibrational cycle so that the resulting collection of interferograms shows how the shape of the vibrating surface changes during the cycle; thus, the interferograms yield information on the shape and amplitude of the vibrational mode or modes.

alt
The Basic Optical Instrument — in both the unmodified and modified versions — is a low-magnification microscope combined with a Michelson interferometer.

The figure schematically illustrates the unmodified and modified versions of the instrument. The magnified image of the specimen is brought to focus in a charge-coupled-device (CCD) camera oriented along a main (vertical) optical axis. In the unmodified instrument, an incandescent lamp generates white light, which is reflected down along the main optical axis by a beam splitter. The microscope objective assembly contains another beam splitter, which divides the illumination into two beams: a reference beam sideways to the main optical axis and an object beam, which continues down along the main optical axis, through the objective lenses, and onto the specimen.

The reference beam is reflected from a flat mirror, then reflected from the beam splitter in the objective assembly back up the main optical axis toward the camera. The object beam is reflected from the specimen, passes up through the beam splitter and the lenses in the objective assembly, and into the camera. Interference between the object and reference beams forms the desired interference pattern on the focal plane of the camera.

The specimen is moved along the main optical axis, passing through the position of zero optical-path-length difference. CCD outputs representing interferograms are acquired and digitized at various increments of optical-path-length difference. Then by use of special-purpose software, the digital interferograms are used to compute the surface profile of the specimen. The unmodified instrument cannot be used to obtain the surface profile of a vibrating specimen because the vibration blurs the interference patterns.

In the modified instrument, the incandescent lamp is replaced by an ultrabright light-emitting diode (LED), which can be driven by pulses at a repetition rate up to 2 MHz. The pulse generator that drives the LED is synchronized to the function generator that drives the vibrations of the specimen. The phase-offset capability of the function generator is used to select the increments of phase for acquiring sets of interferograms.

Because of the viscous drag, many microelectromechanical structures do not vibrate in air at atmospheric pressure. Therefore, in the modified instrument, the specimen is mounted in a small vacuum chamber and observed through a window. To compensate for the optical path length through the window, the modified instrument includes a plate of the same optical thickness as that of the window inserted in path of the reference beam.

With the modified instrument, vibrational displacements along the main optical axis can be measured with resolutions of the order of nanometers. The only fundamental limitation of the modified instrument lies in the competing requirements for stroboscopic illumination: One must generate enough luminous energy during each pulse to obtain an image, while keeping the pulse short enough, to prevent motion blur of the image. "Short enough" as used here is defined with respect to the amplitude and frequency of the vibration and means, specifically, that the displacement during the pulse must be no more than about 1/20 of the wavelength of the light from the LED.

This work was done by Roman Gutierrez, Kirill Shcheglov, and Tony K. Tang of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Physical Sciences category. NPO-20177