Illumination optimization for

optical semiconductor metrology


Bryan M. Barnes*a, Lowell P. Howardb, and Richard M. Silvera

aNational Institute of Standards and Technology, Gaithersburg, MD USA 20899-8212
bPrecera, Inc. PO Box 60, Damascus, MD USA 20872-0060



Uniform sample illumination via Köhler illumination is achieved by establishing a pair of conjugate focal planes; a light source is focused onto the condenser lens back focal plane while the image of the field aperture is focused at the plane of the specimen.  Placement accuracy of these elements along the optical axis will determine the quality of the spatial homogeneity of the illumination in the sample plane.  Imaging is therefore inherently sensitive to the tolerance of this alignment.  Measuring three-dimensional features on semiconductor wafers has demonstrated an additional requirement for angular illumination homogeneity, as this is critical for robust matching between experimental results and numerically modeled results.[1]  We outline our techniques for aligning a custom-built microscope that features a 12 mm diameter conjugate back focal plane, beginning with the establishment of an optical axis based upon the objective lens.  We then illustrate the techniques used to establish the quantitative degree of spatial and angular homogeneity through quantifying the resultant illumination while apertures are scanned across this conjugate back focal plane.  Examples of the correlation between homogeneity and optical metrology are provided.


[1]   J.M. Pedulla, J. Potzick, and R. Silver, “Improving the uncertainty of photomask linewidth measurements,” Proc. SPIE 5375, 317-327 (2004).


Bryan M. Barnes
Mentor: Rick Silver
Precision Engineering Division
Manufacturing Engineering Laboratory

MS 8212  

220/A219 [office], 217/E112 [lab]
x3947 [office], x74378 [lab], (301) 869-0822 (FAX)
Not presently a Sigma Xi member
Category: Engineering