Structural characterization of tantalum nitride films as wet etch stop layer in advanced multiwork function metal gate MOSFETs

As the dimensions of semiconductor devices become smaller, the use of high-k materials as gate oxides has become necessary for controlling gate leakage current. In addition, to achieve higher performance, conventional polysilicon metal gates have been replaced with multistack metal electrodes with engineered work functions. Thus, a suite of devices with different levels of performance and power usage is available. The integration scheme to create such structures involves an additional deposition step and selective wet etching of titanium nitride (TiN) in a complex patterning scheme following the high-k film deposition. Tantalum nitride (TaN) is one of the candidates used as an etch stop layer in a bilayer scheme of TaN/TiN. This prevents any exposure of high-k hafnium oxide (HfO2) gate oxide to wet etch chemistry in the patterning scheme, which could cause damage and/or skimming, resulting in undesirable effects on device characteristics and reliability. In order to accommodate for shrinking trench widths, these bilayer stacks are kept as thin as possible, which makes monitoring the thickness during various patterning steps very challenging. In this study, the authors present an approach to an offline and inline metrology setup. In order to gain insight into basic layer growth and interdiffusion characteristics, they used x-ray photoelectron spectroscopy and high-resolution transmission electron microscopy. They utilized specular x-ray reflectivity as a first principles method, followed by wavelength dispersive x-ray fluorescence to gain insight into cross-wafer variation on blanket wafers. The results were then fed into models developed for the x-ray photoelectron spectroscopy technique, which can be used as an inline method to characterize the composition and thickness of the patterned wafers for both as-deposited and postetching.