Resist and etched line profile characterization using scatterometry

In previous applications scatterometry has shown promise as a metrology for several process measurements. The linewidths of both resist and etched features, and the thicknesses of several underlying film layers, have been accurately characterized using the technique(1). Up until recently these results have been obtained by assuming the features being measured possessed a nominally square profile. However, as metrology tolerances shrink in proportion to device dimensions, errors in the measurement technique due to non-square line profiles could become significant. To test the ability of the scatterometry technique to measure non-square profiles, two models have been developed. The first profile model assumes the top and bottom corners of a resist line can be approximated as a segment of some circle with a given radius. With the center of the circle fixed in space by the overall height of the resist and a nominal linewidth, the sidewall of the line is then modeled as the tangent line that connects the two circles. This particular model can accommodate both overhanging (>90 degrees) and trapezoidal sidewalls (<90 degrees) with just four parameters: the radius of the top and bottom corners, and the nominal top and bottom linewidths. Comparisons between cross-section SEM images and scatterometry profiles using this model will be presented. The second model, which we call the ''stovepipe'' model, is a modified version of a simple trapezoid model and has applications to etched features. In this model an etched line is parameterized by assuming the trapezoidal portion of the sidewall starts at some distance below the top of the line, with the top portion of the line remaining square. In this manner an etched profile can be modeled with four parameters: the overall height of the etched line, the nominal etched linewidth, and the overall height and sidewall angle of the trapezoid layer. Once again, scatterometry profile results in comparison to cross-section SEM images will be presented. The use of both of these models has reduced the difference between scatterometry and SEM CD measurements. For example, the average difference of twelve resist CD measurements, when compared to cross-section SEM measurements, improves from 19.3 to 10.1 nm when the full profile model is incorporated.