Study of rapid thermal annealing on ultra thin high-k HfO2 films properties for nano scaled MOSFET technology

doi:10.1016/j.mee.2011.09.005

Microelectronic Engineering

Volume 91, March 2012, Pages 137-143

https://doi.org/10.1016/j.mee.2011.09.005 Get rights and content

Abstract

The effect of rapid thermal annealing on structural and electrical properties of high-k HfO₂ ultra thin films deposited by rf-sputtering system is investigated. The films properties were investigated for optimum rapid thermal annealing temperature in oxygen and nitrogen ambient, respectively to get the best electrical results as a MOS device structure. Detailed studies of temperature induced annealing effects on the HfO₂/Si interface are done using Fourier Transform Infrared Spectroscopy (FT-IR). The film thickness, composition and microstructure is studied by Ellipsometry, XRD and AFM, respectively, and the effect of annealing on these parameters is shown. The I–V and C–V characteristics of the annealed dielectric film were investigated employing Si/HfO₂/Si MOS capacitor structure. The results showed that the HfO₂/Si stack with rapid thermal annealing (RTA) in nitrogen ambient showed improved physical and electrical performance than with in oxygen. It is shown that RTA improves the interface properties of HfO₂/Si and the densification of HfO₂ ultra thin films. The as deposited films were amorphous and orthorhombic after annealed at 700 °C in nitrogen and oxygen, respectively. We found that the nitrogen annealed samples exhibit a reduced equivalent oxide thickness, interfacial density of states, capacitance–voltage hysteresis and leakage current; additionally it also showed negligible charge trapping under positive voltage bias and temperature stress. The results are presented and discussed.

Graphical abstract

Introduction

High-k dielectric materials have been considered as alternative gate oxides to overcome the scaling limit of SiO₂ due to high tunneling and reliability concerns.

Among the various high-k dielectrics materials HfO₂ is considered as one of the most promising materials [1], [2], [3], [4], [5], [6], [7]. HfO₂ exhibits desirable properties including a high dielectrics constant, high density, large bandgap, and a good thermal stability in contact with silicon relative to the other high-k materials. For ultrathin regime of equivalent oxide thickness (EOT) <1 nm, penetration of oxygen and impurities should be suppressed to maintain low EOT and reduce flatband voltage fluctuation. Many issues such as electrical performance and thermal stability are directly affected by that interface [8], [9], [10]. Other studies on the thermal behavior of HfO₂ reported relatively poor stability and formation of metallic silicide [11], [12], [13], [14]. Those studies report crystallization and morphological changes in the HfO₂ films. Some annealing studies reported changes in the interfacial chemical structure that suggest silicide formation [15]. However, the physical and electrical properties of HfO₂ suffer from its crystallization at high temperature during post deposition annealing, which in turn induces higher leakage current and severe boron penetration issues [16], [17]. High temperature annealing leads to fast diffusion of oxygen through the HfO₂, resulting in the growth of uncontrolled low-k interfacial layers [11]. The uncontrolled low-k layers pose a serious limitation to further scaling of the equivalent oxide thickness (EOT) for HfO₂ gate dielectrics. Nevertheless, several integration challenges remain for these films in terms of their chemical stability, crystallinity and stoichiometry. Variables affecting overall gate stack quality include surface cleaning, specific deposition process (layer thickness, material composition and microstructure, structural defects) and post deposition annealing methodologies. All of these could have an impact on the gate stack quality and electrical properties such as equivalent oxide thickness (EOT), gate leakage current, and transistor characteristics such as the threshold voltage shift.

In this study, ultra thin HfO₂ gate dielectric films were deposited on Si(1 0 0) substrate at room temperature and rapid thermal heat treatment was given in nitrogen and oxygen ambient to study the effect on the structural and electrical properties of HfO₂ thin films.

Section snippets

Experiments

The HfO₂ thin films were deposited by Dielectrics Sputter System (Advanced Processing Technology) on the 4–7 Ω cm p-type Si(1 0 0) wafers at forwarding power of 150 W, base pressure of 1.6 × 10⁻⁵ mbar and operating pressure .016 mbar for 45 s. High purity (99.999 pure) 5 in. HfO₂ sputtering target was used to deposit ultra thin films. All the samples were treated with an RCA preclean process before high-k dielectric deposition. This cleaning process is found to improve the interface and initial growth of

Results and discussion

FT-IR analysis is a reliable and powerful method for characterization of material composition due to rapid and non-destructive measurement. Fig. 1 shows the FT-IR spectra of the HfO₂ ultra thin films samples annealed at 700 °C in N₂ and O₂ ambient for 30 s each. There are three peaks, at 750, 650, and 600 cm⁻¹, attributed to the Hf–O chemical bonds in the spectra, and the peaks at 1100 and 970 cm⁻¹ are assigned to Hf–O–Si stretching vibrations. Comparing the spectra (N₂ and O₂ ambient annealed at

Conclusion

In summary, ultra thin films of HfO₂ were prepared using rf-sputtering. The effects of rapid thermal annealing on the film microstructure, chemical composition and electrical properties in ultra thin HfO₂ films are studied. It is shown that RTA improves the interface properties of HfO₂/Si and the densification of HfO₂ ultra thin films. The as deposited films were amorphous and that were orthorhombic after annealed at 700 °C in nitrogen and oxygen atmosphere. It is found that nitrogen annealed

Acknowledgments

The work was done under CSIR ES scheme. The authors thank Director CEERI and all members of Sensors and Nanotechnology Group (SNG). The authors are grateful to INUP, Electrical Departments, IIT Bombay, for providing facilities for depositions HfO₂ ultra thin film, Rapid Thermal Annealing and electrical characterizations. One of the authors Vikram Singh would like to acknowledge the financial support from CSIR, New Delhi for award of Senior Research Fellowship.

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Study of rapid thermal annealing on ultra thin high-k HfO2 films properties for nano scaled MOSFET technology

Abstract

Graphical abstract

Introduction

Section snippets

Experiments

Results and discussion

Conclusion

Acknowledgments

Microelectron. Eng.

Mater. Sci. Eng. B

Appl. Surf. Sci.

Thin Solid Films

Vacuum

Mater. Sci. Semicond. Process.

Microelectron. Eng.

J. Phys.: Condens. Matter

Appl. Phys. Lett.

J. Mater. Sci. Mater. Electron.

Microelectron. Int.

Appl. Phys. Lett.

Tech. Dig. Int. Electron Devices Meet.

J. Appl. Phys.

Study of rapid thermal annealing on ultra thin high-k HfO₂ films properties for nano scaled MOSFET technology