New tool for the characterisation of diffraction gratings

Photograph of the developed system and the recorded fields at different positions on the grating
Photograph of the developed system and the recorded fields at different positions on the grating

Tool based on Coherent Fourier Scatterometry developed for the characterisation of local and global pitch for UV/VIS/IR diffraction gratings

Diffraction gratings play a key role in many different scientific and technical contexts, such as dimensional metrology, spectroscopy and radiometry. For example, most of optical instrumentation for Earth observation or space research, such as spectral imagers, make use of diffraction gratings as dispersing elements. One major problem for such devices is spectral straylight, which limits the performance of a measurement instrument due to its impact on the accuracy and precision of the measurand.

Diffraction gratings can represent a source of straylight due to the presence of imperfections and local pitch modulation. Typically, the most widely used fast measurement techniques, such as reflectometry, can only provide information on the global pitch, while contact methods, like atomic force microscopy (AFM), can provide local information at the nanometre scale but at the cost of measurement speed. This is a key ingredient not only for Space and Earth Observation instrumentation but also very relevant for the semiconductor industry.

Within the EMRP project Traceable characterisation of thin-film materials for energy applications (ENG53 ThinErgy), scientists at VSL (the Dutch National Metrology Institute) have developed a tool, based on Coherent Fourier Scatterometry, which can be used to determine local and global pitch homogeneity of periodic structures, like diffraction gratings and contact holes. Currently, pitch size from 300 nm and sample size up to 50mm x 50mm can be measured at high speed, with an uncertainty of few nanometres. The system can also be employed to measure thickness, refractive index and absorption of multilayered structures in use in the renewable energy sector, power electronics and coating industry.

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