The fast evolution of microelectronics fabrication technology demands a concurrent development in metrology
capabilities. In recent years, Mueller Matrix (MM) scatterometry has been asserted as a useful tool in
characterizing critical dimensions (CD) in periodical arrays of nanometer-size structures. Specifically, some
symmetry properties of the measured structure can be readily extracted from the MM, allowing effective
isolation of abnormal features. One example is measuring deviations of grating structures from perfect mirror
symmetry, characteristic of faults in the fabrication process.
The most general form of the Muller matrix requires 16 independent measurements, and requires spectral
ellipsometry. However, using some very general assumptions on the reflection properties of the measured
sample, one can reduce this number considerably. Such realistic assumptions are time independence of the
reflection properties, and homogeneity of the sample (i.e., constant reflectivity throughout the measurement
spot), as is the common case in optical CD metrology targets. We show that under these assumptions the
Mueller matrix can be completely measured using spectral reflectometry.
The goal of characterizing asymmetry is then further analyzed, and a new approach for such measurement,
based on spectral reflectometry, is presented. Specifically, using spectral differences metrology (SDM), this
approach is shown to provide a simpler means to measure the same asymmetry-dependent quantity as targeted
today using MM metrology, but requires only two distinct measurements leading to improved throughput.