Profilometer Inspects Microlenses

Brent D. Johnson

The absence of an accepted standard for microlens wafer formats, which may include microlenses ranging from microns to several millimeters in size, has resulted in a demand for profilometers that can satisfy the metrology requirements of characterizing the radius of curvature and the deviation from sphere. They must also accommodate aberrations of microlenses embedded on a variety of wafer configurations.

The MicroXam interference microscope's primary application at Rockwell International is characterizing microlens detector and sensor arrays.

ADE Phase Shift product manager Nabeel Sufi explained that the arrangement of the microlens elements changes from wafer to wafer, and the elements that aren't utilized don't have to be measured.

Haluk Sankur of Rockwell International has been using ADE's MicroXam interference microscope for optical measurement of microlenses and microelectromechanical systems as well as of microprocessor arrays. Although his applications include telecommunications, displays, fiber optic collimators, laser array collimators and color separation for displays, his primary use for the device is characterizing microlens detector and sensor arrays.

Using a CCD camera that produces a low-noise image at 1k x 1k pixels, the device calculates the radius of curvature by measuring a significant central portion of the microlens and fitting the best-fit sphere to its surface. The magnification selected is based on the results. Aberrations and radius of curvature are described by Zernike polynomials, but other parameters may be used to characterize aspheres.

The microscope can also be programmed to determine whether the microlenses on a single wafer meet the manufacturing criteria for various applications. It rejects certain lenses while qualifying others by comparing them with industry-standard statistics that are stored in reports as graphical data.

Sankur said that, in terms of scale uniformity and rapid characterization, the device is fast and accurate and exhibits minimal bias. It also has a variety of objectives for small and large features.