Determination of side wall angle of nanostructures
Using coherent scanning Fourier scatterometry to obtain information about side wall angle based on focusing light onto a structure and obtain the far field as the structure is scanned along the lateral direction.
In the semiconductor industry, the quality of the lithographic processes are evaluated by determining some parameters of specific structures. These structures are often gratings of which the shape is described by geometrical parameters such as period, MidCD, height and side wall angle.
Far field optical scatterometry is a powerful tool that is used in the semiconductor industry to determine these parameters. The technique is based on shinning a light beam onto the structure and recording the far field intensity at various angles. The obtained data is used in combination with a numerical model based on a priori information about the structure, and it has been shown that the parameters can be determined with subwavelength accuracy. But it is also known that, among these parameters, the accuracy on the determination of the side wall angle is the most challenging problem, since this has usually nominal values close to 90 degrees.
In this case, the changes of the far field intensity as the side wall angle is tuned within a few degrees from this nominal value are extremely small and it is very hard to determine it.
Recently, coherent scanning Fourier scatterometry has been introduced as an alternative way to obtain information about side wall angle based on focusing light onto a structure and obtain the far field as the structure is scanned along the lateral direction. In general, the modelling of side wall angle based on numerical methods is difficult to implement because these angles are very steep, and the discretization of the structure may not be accurate enough. A new approach based on an analytical model to study the far field scattering of a cliff and a ridge structure with steep side wall angles has been developed by the group recently.