Deposition, stabilization and characterization of zirconium oxide and hafnium oxide thin films for high k gate dielectrics

Abstract

As the MOS devices continue to scale down in feature size, the gate oxide thickness is approaching the nanometer node. High leakage current densities caused by tunneling is becoming a serious problem. Replacing silicon oxide with a high kappa material as the gate dielectrics is becoming very critical. In recent years, research has been focused on a few promising candidates, such as ZrO₂, HfO₂, Al₂O₃, Ta₂O₅, and some silicates. However, unary metal oxides tend to crystallize at relatively low temperatures (less than 700°C). Crystallized films usually have a very small grain size and high leakage current due to the grain boundaries. The alternatives are high κ oxides which are single crystal or amorphous. Silicates remain amorphous at high temperatures, but have some problems such as phase separation, interface reaction, and lower κ value. In this work, we addressed the crystallization problems of zirconium oxide and hafnium oxide thin films. Both of these two thin films were deposited by DC reactive magnetron sputtering so that very dense films were deposited with little damage. A specially designed system was set up in order to have good control of the deposition process. The crystallization behavior of as-deposited amorphous ZrO₂ and HfO₂ films was studied. It was found that the films tended to have higher crystallization temperature when the films were thinner than a critical thickness of approximately 5 nm. However, it was still well below 900°C. The crystallization temperature was significantly increased by sandwiching the high kappa oxide layer between two silica layers. Ultra thin HfO₂ films of 5nm thickness remained amorphous up to 900°C. This is the highest crystallization temperature which has been reported. The mechanisms for this effect are proposed. Electrical properties of these high kappa dielectric films were also studied. It was found that ultra thin amorphous HfO₂ and ZrO₂ films had superior electrical properties to crystalline films. The leakage current density of ultra thin amorphous films was at least two orders of magnitude lower than that of crystallized films. Amorphous films also showed much less hysteresis in the capacitance-voltage curve than uncapped crystallized films. The mechanisms for the electrical property differences between ultra thin crystalline and amorphous films were studied. Due to successful control of the low dielectric interfacial layer thickness, an effective oxide thickness of 1.2 and 1.4 nm was obtained for HfO₂ and ZrO₂ films, respectively.