Study of HfO2 based high-k gate dielectric thin films grown by pulsed laser deposition

Abstract

The current SiO2 gate dielectric with a relatively low permittivity is reaching its limit of usefulness as the feature size of complementary metal oxide semiconductor (CMOS) devices shrinks to nanometer range. Alternative gate oxide with higher relative permittivity is necessary so that thicker films can be used in the future advanced CMOS technology. This can eliminate the problem associated with low-k SiO2 such as high leakage current density due to direct tunneling. Among many alternative gate dielectrics, HfO2 and its aluminates have received much attention and been considered as promising candidates. Epitaxial yttrium stabilized HfO2 thin films were deposited on p-type (100) Si substrates by pulsed laser deposition at a relatively low substrate temperature of 550C. Transmission electron microscopy observation illustrated a fixed orientation relationship between the epitaxial film and Si, i.e. (100)Si//(100)HfO2 and [001]Si//[001]HfO2. The film/Si interface is not atomically flat and sharp, suggesting possible interfacial reaction and diffusion. The interfacial reaction and diffusion were further confirmed by X-ray photoelectron spectrum (XPS) analysis showing Hf silicate and Hf-Si bonds formation at the interface. The epitaxial growth of the yttrium stabilized HfO2 thin film on bare Si is by a direct growth mechanism without involving the reaction between Hf atoms and SiO2 layer. High frequency capacitance-voltage (C-V) measurement revealed that the relative permittivity of an as-grown 40 A-thick yttrium stabilized HfO2 is about 14 and the equivalent oxide thickness (EOT, here "oxide" refers to SiO2) is 12 A. The leakage current density is 7.0 x10-2 A/cm2 at 1V gate bias voltage. In order to obtain a stable amorphous structure under thermal annealing and suppress the tunneling leakage current, Al2O3 has been selected to alloy with HfO2. Ultrathin amorphous Hf-aluminate (Hf-Al-O) films have been deposited on p-type (100) Si substrates by pulsed-laser deposition using a composite target containing HfO2 and Al2O3 plates. Transmission electron microscopy observation of the Hf-Al-O films showed that the amorphous structure of Hf-A1-O films was stable under rapid thermal annealing at temperatures up to at least 1000C. C-V measurement of a 38 A Hf-AI-O film revealed that the relative permittivity of the film was about 16. The film showed very low leakage current density of 4.6 x 10-3 A/cm2 at 1 V gate bias. The Hf-Al-O film under optimized condition did not show any significant interfacial layer at the interface and an equivalent oxide thickness of less than 10 A has been achieved. The formation of Hf-O and Al-O bonds in the film was revealed by XPS. However, for the films deposited in a high vacuum environment without introducing any gases, islands of Hf suicide formed from interfacial reaction have been observed on the surface of Si substrate. The formation of Hf suicide is attributed to the presence of Al oxide in the films that triggers the reaction between Hf atoms in the amorphous Hf-Al-O films and Si under an oxygen deficient condition. Impact of suicide formation on the electric properties has been studied by means of high-frequency C-V measurements at 1 MHz on the metal-oxide-semiconductor capacitors. Based on our preliminary results on the growth and characterization of HfO2-based thin films, we conclude that Hf-A1-O is very promising as high-k gate dielectric to replace SiO2.