Advanced interferometric profile measurements through refractive media.
Optical profilers are valuable tools for the characterization of microelectromechanical systems (MEMSs). They use phase sifting interferometry (PSI) or vertical scanning interferometry to measure the topography of microscale structures with nanometer resolution. However, for many emerging MEMS applications, the sample needs to be imaged while placed in a liquid or in a package with a glass window. The increased refractive index of the transparent medium degrades the interference image contrast and prevents any measurement of the sample. We report on the modification of a Veeco NT1100 optical profiler to enable PSI measurements through refractive media. This approach can be applied to any other optical profiler with PSI capability. The modification consists in replacing the original illumination source with a custom-built narrow linewidth source, which increases the coherence length of the light and the contrast of the interference image. We present measurements taken with the modified configuration on samples covered with 3 mm water or 500 mum glass, and we compare them to measurements of uncovered samples. We show that the measurement precision is only slightly reduced by the water and glass, and that it is still sufficiently high for typical MEMS applications. The described method can be readily used for measuring through other types and thicknesses of refractive materials.
Koev
Stephan T
ST
MEMS Sensors and Actuators Laboratory (MSAL), Department of Electrical and Computer Engineering, Institute for Systems Research, University of Maryland, College Park, Maryland 20742, USA.
Ghodssi
Reza
R