Specular X-ray reflectivity (SXR) can be used, in the limit of the effective medium approximation (EMA), as a high-resolution shape metrology for periodic patterns on a planar substrate. The EMA means that the density of the solid pattern and the space separating the periodic patterns are averaged together. In this limit the density profile as a function of pattern height obtained by SXR can be used to extract quantitative pattern profile information. Here we explore the limitations of SXR as a pattern shape metrology by studying a series of linear grating structures with periodicities ranging from 300 nm to 16 m. The applicability of the EMA is related to the coherence length of the X-ray source. For our slit-collimated X-ray source, the coherence length in the direction parallel to the long axis of the slit is on the order of 900 nm while the coherence along the main axis of the beam appears to be much greater than 16 m. Limitations of the SXR pattern shape metrology are discussed and examples of determining quantitative pattern profiles provided.
Pub Type: Journals
contact lithography, effective medium approximation, line gratings, nanoimprint lithography, pattern structure, X-ray reflectivity