The ITRS requires the integration of dielectric materials with effective dielectric constant (k) lower than 2.8. This is achieved using porous SiOCH. Unfortunately during integration in the devices, damages are introduced in the low-k layer by CMP. The impact of these damages on the microstructure and the electronic properties are studied using HAADF imaging and Valence Electron Energy Loss Spectroscopy in TEM environment. Results are compared to low-k capped with an etch stop layer.
The physical properties of the materials involved in semiconductor devices are pushed to their limits for each new technology node. One critical example is the requirements on the properties of the insulator filling the shrinking gap between metal lines. The dielectric constant needs to be as small as possible, but the insulator should still be compatible with chip fabrication steps like chemical mechanical polishing (CMP), etching or annealing. Ultra low-k materials based on porous SiCOH compounds are reported as promising candidates to replace the dense SiO2 . In bulk form a reduction of the dielectric constant from 4 to 2.8 is obtained for such materials. The main reasons for achieving lower dielectric constant is the lower polarizability of Si-C or Si-CH3 bonds than Si-O bond. Unfortunately, during standard fabrication processes like CMP, ULK material is susceptible to be damaged, leading to the degradation of the dielectric properties. To minimize these damages an alternative consists in the deposition of a denser oxide layer on the top of the ULK before the CMP treatment. In this article, we compare the microstructure and the insulating properties of CMP and capped ULK at 7 Metal Level in a finished device.
Microstructure and electronic properties of ULK are determined by the combination of HAADF imaging (High Angle Annular Dark Field) and EELS analysis (Electron Energy Loss Spectroscopy) in a TEM environment i.e. FEI TECNAI G2 . Both specimens are prepared by focused ion beam (FIB) milling. First, cross-sections were cut out of a 300 mm wafer using the in-situ lift-out method. Then, the final thinning is performed using low kV gallium at 30 kV, 5 and 2 kV successively, to reduce surface amorphization.
The insulator layer sequence is either SiCN/ULK/capping material/SiCN or SiCN/ULK/SiCN for capped and CMP samples respectively (fig. 1). The relative thicknesses, calculated from low loss spectra recorded in the middle of the two SiCN layers, located be low and above of the low-k layer, are found to be equal. This indicates that the TEM foil thickness across the low-k layer is constant, as expected from FIB thinning. Since contrast variations in HAADF images are proportional to the product of the density, thickness (t) and atomic number (Z 3/2), HAADF imaging contrasts can be thus only attributed to chemical or density effects. Regarding EELS analysis, all experiments are performed in the low energy domain <100 eV, in line-scan mode across the stack. Spectra are recorded with short exposure time to prevent contamination and irradiation effects and are summed to get a final spectrum with a high signal to noise ratio. In this energy range, EELS signal results from individual and collective valence electrons excitations and gives information on chemical as well as physical properties. The zero-loss peak recorded simultaneously to the low-loss spectrum informs about the experimental resolution and the material density. The calculated single scattering distribution (SSD) spectrum is proportional to the product of the matrix element between VB and CB and the Joint Density of States (JDOS) . Hence, almost each structure in the SSD Spectrum low energy region (<10 eV) is related to an electronic transition. In particular, the first intensity jump can be attributed to the band gap.
Microstructure of Capped and CMP ULK
Figure 1 displays HAADF images of capped and CMP sample. In both cases, the interfaces between the different deposited layers can be observed by contrast variations, showing the ability of HAADF imaging to be an accurate metrology tool. In HAADF images the contrasts are reversed in comparison to bright field TEM imaging, heavy materials appear white and light element or porous material dark. The contrast variations across the metal layer sequences reveal that density increases from low-k, to SiCN, to capping material and Cu. In addition, for CMP sample, we observe that ULK is darker at the bottom than at the top, whereas the same contrast is observed across the capped ULK, indicating that CMP step introduces either density or chemical variations. In figure 2 zero-loss peak and plasmon region line-spectra recorded across the capped and CMP ULK are displayed. The shape and the intensity of the zero-loss peak as well as the plasmon region appear identical at any level in the capped ULK layer, indicating that neither significant density nor chemical composition changes occur. On the contrary, for CMP sample, the zero loss and plasmon loss region intensities increase by a factor of 2, from the bottom to the top, but no change is observed neither in the shape nor in the plasmon peak energy. The contrast variation across the CMP low-k layer can be thus attributed to density variation effects. SiOCH compounds are known to be porous materials. Thus a likely interpretation is that the chemical solution used for CMP step, penetrates into ULK and slightly dissolves ULK. Pore volume fraction is thus increases and density decreases .
Band Gap Measurement of ULK
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