Using Ellipsometry for Assessment of TiN Surface Roughness after Plasma Etch

Continuous downscaling of semiconductor devices that enables high-speed operation and miniaturization of modern integrated circuits brings new challenges for semiconductor manufacturing. Conventional materials used for the gate stack polycrystalline Si as a gate electrode and SiO2 as a gate dielectric can no longer comply with stringent requirements of low gate-leakage current and high source–drain drive current. Aggressive scaling of SiO2 results in increased leakage current, while poly-Si gates suffer from depletion and boron diffusion into the channel region. In order to alleviate these problems, SiO2 is being replaced by dielectrics with high dielectric constant 1 high-k dielectrics while polycrystalline Si is substituted by metallic conductors called metal gates. 2 One of the materials used as metal gate is titanium nitride TiN. It is readily available in semiconductor manufacturing because it is already used as a diffusion barrier for copper metallization and as an antireflective coating ARC for aluminum metallization. One TiN deposition method is physical vapor deposition PVD. Historically, the first PVD TiN was prepared by sputtering of Ti target in the presence of N2 ambient. 3,4 No substrate bias is applied to the substrate that is kept at room temperature. Such a process was used to obtain ARC for patterning of Al metallization, and PVD TiN of that type is sometimes referred to as ARC TiN. Later, when PVD TiN started to be used for diffusion-barrier applications, it was found that the topography coverage of ARC TiN is far from perfect. In order to improve it, a new deposition technique called ionized metal plasma IMP was developed. The main difference between ARC and IMP is that in the IMP source the metal is ionized and a bias is applied to the substrate, resulting in improved topography coverage. 5 The substrate temperature can be varied in the limits of 0–450°C. It is known that the microstructure of PVD TiN depends on power settings and substrate temperature during the deposition process. 6 Usually it has a columnar structure with columns oriented perpendicular to the substrate. When no bias is applied to the substrate and deposition temperature is relatively low close to room temperature, which is the case for ARC TiN, the columns are oriented in 111 direction when the film is relatively thick thicker that several tens of nonometers. 7,8 Application of bias typical for IMP TiN changes the column orientation to 200 or in some cases can result in almost amorphous layers.