October 1, 2006

Update on the X27A Micro-spectroscopy Facility Beamline

We are now into the third cycle of general and contributing user operations at NSLS hard x-ray micro-spectroscopy facility beamline X27A. A smooth transition from commissioning to operations during the fall of 2005 provided immediate implementation of a wide and diverse research program involving an extensive range of institutions: Stony Brook University, Rutgers University, BNL Environmental Sciences Department, Miami University, Institute of Nuclear Physics (PAN) Poland, University of Western Ontario, Natural Resources Canada and IMEC of Belgium. Ongoing research areas include the study of manganese and arsenic speciation/distribution in a range of environmental sensitive samples; the nature of titanium incorporation through applied creams into human skin layers; plutonium and uranium distribution/speciation in contaminated soils; polarization dependent thorium EXAFS measurements on small apatite single-crystals; understanding strontium incorporation in osteoporotic bone; and determining trace-metal distribution in diseased brain tissue. The success of implementing these research areas, and the high-quality data being collected at this facility, is largely due to the good stability of the NSLS x-ray ring, which is especially important for microprobe beamlines. Further details and an extensive description of the X27A micro-spectroscopy facility can be found in Nucl. Instrum. & Methods in Phys. Res. A 562(1) 487-494, 2006 and at http://www.nsls.bnl.gov/beamlines/x27a/.

Figure 1. X-ray fluorescence line scans across 10um-wide copper interconnects spaced at (A) 40 um and (B) 90 um. (C) Cu Kd fluorescence image of a pad structure.

Figure 2. Ta L2 absorption-edge XANES on Ta2N (5 nm)/Ta (20 nm) blanket film and the Ta2N barrier on a Cu interconnect patterned sample, both on and off the interconnect lines.

An example of an area of research initiated at X27A is the investigation of tantalum and tungsten thin-film diffusion barriers that are used in copper interconnect technology. Using the x-ray microbeam, buried thin-film layers beneath copper are investigated using fluorescence x-ray absorption spectroscopy. The ability to characterize these films, which are often amorphous and cannot be studied using x-ray micro-diffraction, is an important area of applied research within the semiconductor industry. Currently, preliminary experiments on thin-film interconnect test structures have been conducted and analyzed, and these encouraging results have initialized the fabrication of ‘real’ systems for future in-situ measurements. Preliminary measurements on passivated 10 m wide, 120 nm thick Cu interconnects, separated by 40 m and 90 m ‘field’ regions, and with a 4 nm thin ALD Ta₃N₄ barrier layer have been performed. Cu K fluorescence recorded at an incident x-ray energy of 9.5 keV, and Ta L₁ fluorescence recorded at the peak of the Ta white-line at ~11.142 keV, are shown in Figure 1. The Ta L₂ XANES on a PVD Ta₂N (5 nm)/Ta (20 nm) blanket film and a 4nm ALD Ta₃N₄ barrier used in a copper interconnect test structure are shown in Figure 2. As can be seen, the nitrogen content within the Ta₂N barriers is clearly evidenced by the spectral signature of the near-edge region and the shift in the absorption edge towards higher x-ray energies, compared to the Ta₂N (5 nm)/Ta(20 nm) blanket film, which is of predominantly Ta character.

ARTICLE BY: James Ablett