Fluorescence Imager Enhances E-Beam Platforms

X-ray fluorescence has been a useful metrology and inspection resource for many years. By analyzing X-rays emitted by certain materials as a result of the energy pumped into them by e-beam platforms (SEMs and TEMs), it is possible to derive useful data. Fluorescence X-rays are of a specific energy that uniquely relates to the element that emits them, creating a spectroscopic fingerprint; until now, however, it had not been possible to produce a most useful metrology and inspection resource: an X-ray fluorescence image.

Xradia Inc . (Concord, Calif.) has developed an imaging tool that fills this need. Its nanoXFi X-ray fluorescence imaging system works in conjunction with e-beam sources, such as SEMs, TEMs or e-beam metrology tools. The fluorescence imager provides those platforms with the capability to image X-ray-emitting materials. It non-destructively captures and images X-ray fluorescence, creating visual reconstructions of what those materials look like, providing a detailed view of subsurface structures of interest. This enables the production of virtual spatial maps or images using element-specific fluorescence resulting from regular e-beam-based metrology applications. Currently, the unit is unique in its capability to generate detailed visual maps of X-ray fluorescing materials.

This capability could not be had sooner, because the X-ray optics' commercial availability — components that collect and focus X-rays through diffraction rather than refraction — is relatively new. X-ray optics have been fairly confined to R&D uses in some national laboratories, where it has been applied to synchrotron radiation sources to do high-resolution X-ray microscopy. Also, until recently, detectors have not been sensitive enough to be used with these optical components to collect images by using fluorescence. The improvement in the optics (both in efficiency and throughput) and detector design, and the conversion efficiency of X-ray photons to visible-light photons to image onto a CCD have made it practical to build this instrument.

In a scan using an e-probe and spectrometer (left), spatial resolution is limited by electron scattering, fine features cannot be resolved, and depth information is lost. In imaging using the zone-plate lens (right), the probe size determines the field of view and spatial resolution limited by the optics, and depth information is retained. (Source: Xradia)

The fluorescence imager is a passive device. It lacks any source of light or radiation of its own. It is a tube ~70 cm long and 10 cm across, with a lens or "zone-plate" optical focusing element that is positioned within a few millimeters to the sample under probe and collects fluorescence emanating from surface and subsurface structures. It focuses the resulting fluorescence on a scintillator and then a CCD camera. A motor drive mechanism incorporated into the cylinder sets the lens position at different focal points, enabling the mapping of different elements that fluoresce under an e-beam such as copper, gold, silicon, aluminum or nitrogen. Anything in the periodic table with a number higher than Z4 can be mapped and imaged in this way.

The tool attaches at an angle, generally 45° to normal, and captures X-ray fluorescence underneath an e-beam probe, resolving <80 nm features. Its use is dedicated to the area of subsurface phenomena of interest — defects — at the wafer and die level, which can then be visualized either in process or offline in a non-destructive, non-invasive, fairly rapid methodology.

In visualizing what is beneath the probe, the fluorescence imager acts as an enhancement to conventional e-beam platforms using spectroscopic techniques to measure the quantitative and qualitative content of materials under the probe. Since the zone-plate lens is a chromatic optical component, it focuses different wavelengths (or energies) at different points. In use, the lens focuses one wavelength at a time — meaning one element at a time — onto the CCD. This enables the technology's most compelling attribute: its capability to visually isolate a particular feature within a wafer, such as copper interconnects, and image it in more than one layer at a time, either on or below the surface.

Metrology and inspection equipment OEMs, as well as users, understand the advantage of being capable to image, in an element-specific manner, problems like copper voids, defects and particle contaminants below the chip's surface in a non-invasive, non-destructive manner. A number of different applications are yet to be added, including layer-thickness analysis and layer characterization in terms of uniformity.

Work is underway to develop them. Currently, the system is designed for use with other platforms and not as a standalone system, and is being integrated into e-beam systems by some leading OEMs.

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