Figure 3 displays the calculated SSD spectra representative for ULK layer in capped sample and for ULK at the top and bottomregions in CMP sample. For each one of these spectra two significant intensity jumps are observed within the 0-10 eV range. For the capped ULK, the first jump is located at 2.4 eV and the second at 8.5 eV while for CMP ULK, the first is located at 1.1 ± 0.1 eV and the second at 8 and 8.5 eV depending on the acquisition place. To valid the method, the same band gap determination is applied to spectra recorded from the well known phases present in the stack i.e. SiCN and amorphous-SiO2. For both, band gap and low-loss structures energies are in agreement with values reported in the literature. The presence of any artefacts in the ULK SSD spectra due to Cerenkov effects or surface coupling effects can be excluded , since refractive index of SiCOH compound is reported lower than 1.4 and TEM foil thickness is measured to be 55 +/-5 nm. This confirms the reliability of our results. Comparing ULK, SiO2 and capping material SSD spectra, we find that they are not very different in the energy range above 10 eV, where plasmon contributions dominate the spectrum. The typical structures located at 10.5, 14.6 and 18.1 eV observed in am-SiO2 spectrum are also particularly well reproduced, indicating that SiOCH electronic behaviour is close to the SiO2 one. The only difference is that the SiOCH plasmon energy position is shifted about 1 eV to lower energy. This is in agreement with the expected chemical composition effects resulting to C and H atoms addition to SiO2. The interpretation of the spectra in the energy range below 10 eV is less straight forward. The signatures located at 8 and 8.5 eV for CMP and capped ULK are naturally identified as the band gap since it is the case for the intensity jump in am-SiO2 located at 8.9 eV. While as expected, the intensity falls down to zero in am-SiO2 for energy lower than 8.9 eV, a surprisingly significant signal remains in this energy range from 1.1 to 8 eV and 2.4 to 8.5 eV for CMP and capped ULK respectively.
Taking account of these signal intensity level, it would be unrealistic to attribute it to band tail effects.
The most probable explanation to electron states in the band gap region, are dangling bonds created at the surfaces of the pores in the ULK. Therefore, to our knowledge no calculations supporting these assumptions have actually been carried out. After processing each individual spectrum of line-scans across capped and CMP ULK, a mapping of the band gap and defects signatures as a function of the depth can be established (fig. 4). While in the capped ULK both signatures are constant across the layer, a significant decrease of the band gap from the bottom to top occurs for the CMP ULK. Moreover, the defect signatures energy position is always about twice lower in CMP than in capped ULK, independently of the location in the layer, like as this signal was not impacted by the density variations. Additional experiments are carried out to shine light on this result.
The impact of CMP on ULK material in a state of the art device is investigated using HAADF imaging and EELS in HR-TEM. STEM-EELS line profiles allow an accurate mapping of three fundamental features of ULK material: density, band gap and electronic defect signatures. We have first proven that the capping layer plays its protection role since no densification variations are detected and a flat band gap profile is observed across the capped ULK. In the contrary, a significant modification of the contrasts and electronic properties are observed in the CMP ULK layer. As a consequence, a negative impact on the dielectric constant of ULK unprotected before CMP is thus expected.
 Maex K. et al.: J. Appl. Phys. 93, 8793-8800 (2003)
 Egerton R. F.: Electron Energy Loss Spectroscopy in Electron Microscope, 2nd Edition, Plenum Press, 131-194 (1996)
 Miranda, P. A. et al.: Microelectronics and Electron Devices, 2004 IEEE Workshop, 85-88 (2004)
 Erni R. et al.: Ultramicroscopy 108, 84-89 (2008)
Dr. Marie Cheynet, Grenoble University - CNRS, SIMaP-PHELMA, France (corresponding author)
Dr. Fabien Volpi, Grenoble University - INP, SIMaP, France
Dr. Simone Pokrant, Product manager, A Carl Zeiss SMT AG Company, Oberkochen, Germany
Dr. Roland Pantel
Dr. Mohammed Aimadedinne
Dr. Vincent Arnal, St. Microelectronics, Crolles, France
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