Back end of line metrology control applications using scatterometry

Abstract

Scatterometry is a novel metrology approach for process control that has recently been gaining more momentum in microlithography applications. The method can simultaneously measure Critical Dimension (CD), Side Wall Angle (SWA), and thickness of more than one layer. It analyzes the scattered and diffracted light from a periodic array of lines or holes that represent the surface structure of the measured sample. Scatterometry provides a non-destructive technique offering high precision and stability along with high tool-uptime performance. As such, it offers an excellent approach for real-time high volume production control with significant advantages as compared to traditional technologies such as CD-SEM and Profilometry. As the structure dimension shrinks considerably, producing high precision results becomes more critical. To date, reports on the deployment of scatterometry in real production environment have focused on Front End of Line (FEOL) applications such as STI and Gate. However, Back End of Line (BEOL) process control has not been widely reported. In this work, we will discuss the results of our study specifically for metal trench and contact layer on both patterned and etched wafers for 65nm technology node. We will also report the comparison between Scatterometry results to Critical Dimension Scanning Electron Microscope (CD-SEM) and Atomic Force Microscope (AFM). Finally we will provide a statistical analysis of our scatterometry results including precision, fleet precision, and TMU analysis. In contrast to the relatively simple stacks that comprise a FEOL structure, BEOL layers are typically complex structures with a large number of underlying layers. Generation of simulated scatterometry signatures that constitute a reference library for complex structures can require long computational times and result in large file sizes. To mitigate the computational overhead, it is necessary to intelligently decide which parameters to fix and which to vary. An additional complication is presented due to similarities in the optical properties of BEOL stack materials, which can introduce potential for parameter cross-correlation in the measurement. We will discuss methodologies for optimally selecting parameters to be fixed or varied to minimize these effects.