Poly-Si/TiN/HfO2 gate stack etching in high-density plasmas.

The authors have investigated the dry etch mechanisms of complex poly-Si/TiN/HfO₂ gate stacks and the issues that are correlated with the introduction of a thin metal layer in the gate stack. Based on atomic force microscopy (AFM) and scanning electron microscope measurements, they will first show that a mixture of HBr and Cl₂ at low rf bias power is required to successfully pattern the TiN layer without damaging the HfO₂ gate oxide. Second, it is demonstrated that the introduction of a metal layer in the gate stack prevents charging effects during the last etching steps of the silicon part of the gate. Transmission electron microscope measurements and x-ray photoelectron spectroscopy analyses of the gate sidewalls show that the thickness of the silicon sidewall passivation layer decreases during the O₂ free metal etching step potentially inducing silicon gate profile distortion such as notch. However, the notch can be eliminated by etching the Si/TiN gate in a single step process instead of stopping at the TiN surface. Finally, AFM measurements show that during the TiN etching step, a low rf bias power is required to prevent damage (punching through) of the HfO₂ layer. However, even under these conditions, a significant silicon recess (oxidation of the c-Si beneath the HfO₂ layer) is observed even if TiN is etched in an O₂-free chemistry. [ABSTRACT FROM AUTHOR]