Small angle x-ray scattering industrial overlay metrology solutions for 7nm nodes and beyond

Through this work, we aim to give an overview of what is doable with Small Angles X-ray Scattering (SAXS) in the future of overlay metrology and position its role in the production of 7 nm technologies and below. With a metrology budget below 0.35 nm as foreseen by the International Roadmap for Devices and Systems (IRDS)¹ and the reduction of the components dimensions, current optical metrology techniques Image Based Overlay (IBO) and Diffraction Based Overlay (DBO) targets will lack of representativeness. Probing smaller structures that are closer to the real processed technologies with a sub-nanometric resolution and precision becomes necessary. SAXS metrology tools appear to be promising solutions thanks to the high resolution conferred by X-rays sub-nm wavelength. We will review three different methods for overlay measurement with SAXS. The first method consists of a full reconstruction of the in-depth profile of the measured stack. Although efficient, this method requires the acquisition of a large set of data, which translates as hour-scale measurement times on structures others than High Aspect Ratio (HAR) ones. The two other methods, respectively based on Veldman et al. patent and Zhang et al. publication^{2, 3} will also be discussed. These methods present the advantage of exploiting the asymmetrical signature of overlay in the SAXS intensity pattern with a limited modeling and reduced data acquisition time. We tested each method on the same set of samples and will discuss their respective strengths and drawbacks, as well as their potential for industrial metrology control.