Sensitivity analysis of optical scatterometry technique for high aspect ratio trench measurement

For CMOS image sensors fabrication, deep trenches are commonly incorporated in the device to isolate the individual pixel one another within the pixel matrix. These etched structures typically exhibit a high aspect ratio of 1:20 and controlling such narrow and deep object is a challenge for inline metrology. In a manufacturing environment, the preferred method for trench height measurements remains the optical scatterometry (OCD) technique as being very sensitive and reliable. Still, it requires time and resources for model construction and validation. It appears then that an analysis of its predicted sensitivity could be a valuable pre-step before starting any activity on large periodical objects where OCD sensitivity can reach its limits. In this study, we tested this approach for deep trench structures with CD dimension in the range of 100nm to 1400nm and the depth from 100 nm to 5 mu m. The periodicity (pitch) was fixed at CD*2. At first, 3D Mueller scatterometry signatures were modelled selecting spectroscopic ellipsometry acquisition configurations according to industrial most common ones. Thanks to an optimized RCWA (Rigorous Coupled Wave Analysis) code developed inhouse, calculation durations were reduced enough to allow massive data generation. By implementing a sensitivity analysis approach that uses Sobol coefficients, the sensitivity of the OCD metrology technique is here evaluated for each CD and depth values. More particulary, it will be illustrated by a CD range of.10% of 350nm and with the depth of the trench varying from 100nm up to 5 mu m. As a results, a sensitivity frontier can be estimated at around 3 mu m, a critical depth value above which OCD in the given configuration is no more sensitive to the metrics determination. Such observation will be further discussed by analysis of convergence evaluation.