Lateral error reduction in the 3D characterization of deep MOEMS devices using white light interference microscopy

White light scanning interference microscopy, with its high axial resolution, is particularly useful for the rapid 3D characterization of MOEMS micro-systems. Although this technique can be used for submicron critical dimension measurement on micron high microelectronic structures, recent tests using a standard system have revealed errors of up to 3 µm in the measurement of lateral position of deep square steps. Thus the 2 µm wide, 75 µm deep teeth of an electrostatic comb structure in a FT MOEMS spectrometer were measured to be nearly 7 µm wide using a Mirau interference objective with the aperture diaphragm of the illumination system fully open in white light. Tests under different conditions show that the error is greatest for the Mirau objective, with the aperture diaphragm fully open at longer wavelengths. In addition, the location of the centre of such structures can vary by up to 1 µm depending on the degree of reference mirror tilt. Investigations of the XZ images of square steps have revealed the presence of "ghost" fringes resulting from diffraction and the conical illumination used. The errors in edge position can be reduced using a Linnik type objective with the aperture diaphragm closed down using shorter wavelength light.