Current Status of CDSAXS: Is it Fab-Ready?

摘要

The semiconductor industry continues to follow Moore's Law with continuously shrinking device dimension sizes. The 14 nm node is now in production and the 10 nm node is rapidly under development. In addition to the substantial challenges of simply fabricating these tiny structures, the current suite of metrology tools are also running against fundamental limits. Current production fabs use optical scatterometry (OCD) for dimensional metrology to track the size and shape of their tiny nanostructures. The three dimensional nature of the latest finFET devices and shrinking sizes result in parameter correlations and size dependent refractive index. Critical dimension small angle X-ray scattering (CDSAXS) is a potential replacement metrology tool that uses X-rays instead of the visible and ultraviolet light used in OCD. The much smaller wavelength of X-rays results in a much smaller diffractive resolution limit. CDSAXS is a variable angle, transmission SAXS measurement where the X-rays go through the entire wafer. It is essentially single crystal diffraction where the lattice is the periodic grating and the individual lines/vias/posts are the atoms. The scattering pattern is solved to determine the size and shape of the periodic features that created it. CDSAXS results in an ensemble average structure and is not sensitive to low concentration defects. Most reported measurements of CDSAXS were done at a synchrotron to take advantage of the much higher X-ray flux. Moving CDSAXS to the laboratory or fab will require the development of new high brightness, compact X-ray sources. Current compact X-ray sources do not have sufficient brightness for a high throughput measurement. The following section will discuss the current status of CDSAXS and discuss the prospects for higher brightness X-ray sources. We will also discuss the required performance metrics for an X-ray source to provide sufficient resolution in a short enough time. Synchrotron measurements have demonstrated that CDSAXS fundamentally works, the primary limitation is getting enough photons from a compact source to make it a high throughput measurement.