Development of Nano-Pattern Recognition and Correlation Technique for Deformation Measurement of Nano-Scale Structures
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An imperative need exists for deformation data from interconnects of silicon devices. The need for nano-scale measurements becomes more urgent as the interconnect technology approaches the 50 nm node and beyond. The reliability of devices is determined largely by thermal and mechanical deformations of interconnect layers during manufacturing and operation. These are inferred by computational analysis, but informed physical analysis is vital to measure the variables and to guide and verify the computations. Deformation measurements are needed urgently in the nanometer range. What is needed is in-plane displacement measurements that are accurate within a fraction of nanometers, together with sub-micron spatial resolution. In recent years, several techniques have been proposed to document nano-scale deformations. They include electron-beam moiré (EBM), nano-scale moiré interferometry, SEM/TEM/AFM digital image correlation (DIC), and speckle interferometry with electron microscopy (SIEM). None of the existing techniques provide both the accuracy/sensitivity (sub-nanometer) and spatial resolution (sub-micron), which are required for the analysis of nanostructures. The objective of this thesis is to develop a new deformation measurement technique to cope with the limitations of each existing technique: A hybrid method is proposed to achieve the goal. The proposed method called Nano-Pattern Recognition and Correlation Technique (N-PRCT) uses regularly oriented nano-scale structures that are fabricated on the surface of the specimen. After obtaining the SEM pictures of patterns on the region of interest before and after loading (deformation), the conventional low-pass filter combined with a de-blur filter (Wiener Filter) are applied to eliminate the noise during SEM imaging effectively. A unique practice of E-beam lithography is proposed and implemented to fabricate regularly oriented patterns required for the N-PRCT technique using PMMA as an E-beam resist. The proposed scheme utilizes the standard SEM for imaging to fabricate the patterns without the need of specially designed E-Beam lithography system, which makes the implementation of N-PRCT practical. Yet, the proposed procedure can produce gauge lengths (approximately 150 nm) than those produced by a commercial E-beam lithography system The proposed method is used to determine the thermally-induced deformations of a passivation layer in a flip-chip package. The regular patterns (115 nm in diameter) are produced on the polished cross-section, and the package is subjected to a thermal loading inside SEM using a specially designed thermal conduction stage. Thermal deformations with the displacement measurement accuracy of less than 0.1 nm are obtained in a field of view of 7 μm. The results show a shear strain concentration at the interface between the passivation layer and the adjacent metal pad.