Thermal stability of high-k layers

Thermal stability of amorphous phases in various high-k layers (Al 2 O 3 , ZrO 2 , HfO 2 , ZrAlO x , HfAlO x and HfSiO x ) and the phase transformation of crystalline ZrO 2 and HfO 2 were studied experimentally, as functions of surface preparation, deposition conditions, material composition and post deposition thermal treatment. It is found that pure ZrO 2 and HfO 2 show relatively low crystallization onset temperatures. The crystalline ZrO 2 or HfO 2 phases are tetragonal or monoclinic, depending on the layer thickness. The phase transformation of metastable t-phase into stable m-phase has been observed in ZrO 2 and HfO 2 . Crystallization behavior of Al 2 O 3 depends on the surface preparation of the substrate. ALCVD grown Al 2 O 3 layers on an oxide-based surface remain amorphous after 1100°C spike annealing, while those on HF-last surface crystallize at temperatures around 800°C. Alloying Al 2 O 3 into ZrO 2 and HfO 2 can improve their resistance to crystallization under thermal exposure. The kinetics of the crystallization in the alloys can be described by linear TTT curves. Hf-aluminates show better thermal stability than Zr-aluminates. A defect model relative to the phase transformation is discussed, based on the above observations.