Figure 1: Xradia nanoXFi x-ray fluorescence imager installed on a JEOL JXA-8600 scanning electron microscope. The nanoXFi attaches to one of the ports of the SEM and images x-ray fluorescence radiation produced by the electron beam of the SEM. It has very high elemental sensitivity and sub-80nm spatial resolution.
Figure 3: The observation angle (take-off angle) can be easily adjusted by tilting the sample in the electron microscope. This enables the separate observation of different layers of the copper interconnect structures. This figure shows a copper image taken of the same Copper IC chip at a 40 deg x-ray take off angle showing some identified defects introduced during CMP process. The additional loss of material (red arrow) and leakage of copper (yellow arrow) into the dielectric are highlighted.
Die-level metal interconnects are prone to manufacturing problems, such as voiding in vias and trenches, and CMP-induced features such as dishing, erosion and incomplete metal removal, leading to shorts. The characterization of such defects is challenging, because they are most often below the surface and require cross sectioning by focused ion beam for electron microscopy analysis.
Xradia’s nanoXFi x-ray fluorescence imager offers an attractive non-destructive alternative analysis method by selectively imaging the interconnect metal at high spatial resolution, even for buried defects up to 2um deep. The nanoXFi functions as an x-ray spectrometer similar to an EDS spectrometer, but is able to image the origin of the x-ray fluorescence (structures of the emitting material) at sub-80nm resolution. The fluorescence imager takes advantage of the highly chromatic properties of it’s x-ray focusing optics to image just one wavelength or one element at a time. Different wavelengths and therefore elements, can be quickly and easily selected, thereby allowing multi-element analysis within the same field of view.
This material is based upon work supported by National Science Foundation under Grant No. 0512910 and No. 0620578 "Development of an Imaging X-ray Spectrometer". Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
Figure 2: One application of the nanoXFi is to image the Cu-La x-ray fluorescence line of copper IC interconnect structures. The figure shows a Cu image of three layers of copper metallization at a 90-degree take-off angle. The copper features appear bright, the dielectric dark. The brightness of relative brightness of vias and other features, indicate the level of copper content.
Figure 4a and 4b: The nanoXFi can be tuned to image different elements of the same sample area. A three layer copper IC chip was first imaged for copper structures shown in figure 4a. The copper structures appear bright and the surrounding dielectric appears dark. In figure 4b, the nanoXFi was configured to imaging the surrounding dielectric with SiKa x-ray lines (1.74KeV). The copper structures now shadow the Si Ka lines and appear dark. This method, called "backlighting" mode, can be used to study any structure other than Copper (such as tungsten, tantalum, tungsten/titanium interconnects etc).