Ellipsometry is a traditional method of monitoring and measuring the thickness of silicon dioxide laid over silicon substrate, in the semiconductor industry.
However, it is uncertain that if ellipsometer calibrations for thick layers can be extended down to the ultra-thin measurements, considering the growing trend to reduce the gate oxide thickness to values below 2nm.
Also, ellipsometers may not be able to differentiate the oxide thickness variations through a wafer or any changes in carbonaceous/water contamination. The use of more complex, multilayered materials instead of silicon dioxide as the principal gate dielectric impose problems to the ellipsometry as it is difficult to characterize buried layers.
Overview of X-ray Photoelectron Spectroscopy (XPS) and Angle Resolved XPS (ARXPS)
Unlike ellipsometry, X-ray Photoelectron Spectroscopy (XPS) can measure the thickness of silicon oxide through the wafer without any interference from carbonaceous/water contamination. This tool also provides detailed investigation of complex gate dielectric materials, as the elemental specificity of XPS enables characterization of all elements and chemical states within the XPS sampling depth.
With the combination of angle measurements i.e. using angle resolved XPS (ARXPS) the elemental and chemical state distributions can be characterized in the gate material corresponding to the depth, thereby providing non-destructive depth profiling. Figures 1 and 2 show the 2D/3D representations of oxide thickness on 200mm wafer from ARXPS data, respectively.
Figure 1. 2D representation of oxide thickness on 200 mm wafer from ARXPS data
Figure 2. 3D representation of oxide thickness on 200 mm wafer from ARXPS data
Using Theta 300 for Mapping Oxide Profile
From the ellipsometric analysis of a 200mm wafer segment with a 4nm oxide, a 0.2nm of oxide ring that is thicker than the wafer’s central region was observed. Using the Thermo Scientific Theta 300 instrument, true features of the oxide profile have been verified. Theta 300 is capable of producing maps that are identical to ellipsometry, for 300mm wafers with 0.01nm thickness precision.
This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – Materials & Structural Analysis.
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