Elsevier

Applied Surface Science

Volume 257, Issue 6, 1 January 2011, Pages 2197-2202
Applied Surface Science

Growth, microstructure and electrical properties of sputter-deposited hafnium oxide (HfO2) thin films grown using a HfO2 ceramic target

https://doi.org/10.1016/j.apsusc.2010.09.072Get rights and content

Abstract

Hafnium oxide (HfO2) thin films have been made by radio-frequency (rf) magnetron-sputtering onto Si(1 0 0) substrates under varying growth temperature (Ts). HfO2 ceramic target has been employed for sputtering while varying the Ts from room temperature to 500 °C during deposition. The effect of Ts on the growth and microstructure of deposited HfO2 films has been studied using grazing incidence X-ray diffraction (GIXRD), and high-resolution scanning electron microscopy (HR-SEM) coupled with energy dispersive X-ray spectrometry (EDS). The results indicate that the effect of Ts is significant on the growth, surface and interface structure, morphology and chemical composition of the HfO2 films. Structural characterization indicates that the HfO2 films grown at Ts < 200 °C are amorphous while films grown at Ts > 200 °C are nanocrystalline. An amorphous-to-crystalline transition occurs at Ts = 200 °C. Nanocrystalline HfO2 films crystallized in a monoclinic structure with a (−1 1 1) orientation. An interface layer (IL) formation occurs due to reaction at the HfO2–Si interface for HfO2 films deposited at Ts > 200 °C. The thickness of IL increases with increasing Ts. EDS at the HfO2–Si cross-section indicate that the IL is a (Hf, Si)–O compound. The electrical characterization using capacitance–voltage measurements indicate that the dielectric constant decreases from 25 to 16 with increasing Ts. The current–voltage characteristics indicate that the leakage current increases significantly with increasing Ts due to increased ILs.

Introduction

Hafnium oxide (HfO2) is a high temperature refractory material with excellent physical, electronic and chemical properties [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. HfO2 is one of the promising candidates fulfilling the requirements of the desired high-k materials. HfO2 has a dielectric constant of approximately 25 [3]; a relatively high bandgap of 5.68 eV [4] and a high density (9.68 g/cm3) that makes it resistive to impurity diffusion [5]. Good thermal stability with Si is one of the advantages of HfO2 over other high-k materials such as ZrO2 [6].

HfO2 films have been studied by several research groups in the most recent years. The earlier reports on HfO2 films grown by different deposition techniques such as atomic layer deposition (ALD), pulsed laser deposition, electron beam evaporation and sputtering to deposit HfO2 indicate the formation of an undesirable interfacial layer (IL) between the oxide and the Si substrate [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]. Formation of ILs at the HfO2–Si interface and their chemical nature is critical to the performance of MOSFETs. ILs can modify the band offset between HfO2 and Si and introduce defects that can potentially trap charges and affect reliability of devices [11]. In addition, ILs containing either SiO2 or a compositional mixture of Hf–Si–O exhibit a lower dielectric constant and therefore affects the high dielectric constant of HfO2 film [12]. Although HfO2 is very stable to form silicide interfaces, it shows propensity to form silicate interfaces [13]. A comprehensive understanding of the ILs formation and suppression is, therefore, needed for utilizing HfO2 in both electronic and optical technologies. IL formation at the HfO2–Si interface comes from re-oxidation steps in the MOSFET fabrication processes [14] and during annealing treatments [15], [16] that form a silicate type layer. Different methods have been proposed to eliminate the ILs and their associated effects. For example, deposition of a thin Hf film on top of Si substrate prior to HfO2 deposition found to be effective to prevent oxygen diffusion to the Si substrate [17], [18] and nitradation of Si substrate before oxidation was found to restrict the silicate IL formation [19]. However, a detailed understanding of the effect of Ts on the ILs for HfO2 films on Si is still a compelling requirement. Specifically, the information on the chemical nature of the ILs, quantitative data, and elementary models to predict the mechanisms at the nanoscale are important. Furthermore, the structural and electronic/optoelectronic properties as a function of thermodynamic variables, such as temperature and pressure, are also important for the other reason that HfO2 exhibits various polymorphs (cubic, tetragonal and monoclinic phases) depending on the preparation conditions [20], [21], [22]. The present work, therefore, has been performed on the radio-frequency (rf) magnetron sputter-deposited HfO2 thin films grown on to the n-type Si(1 0 0) substrates by varying substrate temperature (Ts) in the range from room temperature (RT) to 500 °C. Films were fabricated using a direct deposition from a HfO2 ceramic target. The effect of Ts on the growth, crystal-structure, ILs formation at the oxide–Si interface and their chemical nature, surface morphology of HfO2 thin films is investigated using grazing incidence X-ray diffraction (GIXRD) and high-resolution scanning electron microscopy (HR-SEM) coupled with energy-dispersive X-ray spectrometry (EDS). The current–voltage (IV) and capacitance–voltage (CV) measurements were performed to probe the electrical characteristics. The results obtained are presented and discussed in this paper.

Section snippets

Experimental

HfO2 films were grown using sputter-deposition. A 2″ HfO2 target (99.99%) was used for sputtering. The substrates were (1 0 0) p-type silicon (Si) wafers with a resistance of 10 Ω. The substrates were cleaned using the standard RCA cleaning process to remove organic residues and metal ions. Then, native silicon dioxide (SiO2) was etched from substrates just before deposition with buffered oxide etch 6:1 for 5 min. Samples were kept under vacuum right after native oxide removal. Besides removing the

Results

The GIXRD curves obtained for HfO2 films as a function of Ts are shown in Fig. 1. HfO2 films grown at room temperature did not show any peaks; the GIXRD curves (not shown) are rather broad indicating the amorphous nature of the films. The GIXRD spectra of HfO2 samples grown at Ts = 200–500 °C (Fig. 1) indicate that films are polycrystalline and oriented. A comparison of the GIXRD curves obtained for HfO2 films with the monoclinic HfO2 bulk (green lines) is also presented in Fig. 1. The appearance

Discussion

The emphasis in this work is to elucidate the effect of Ts, an important thermodynamic variable, on the growth and microstructure and correlate the dielectric constant to the structure of HfO2 films made by sputtering. A discussion of the GIXRD and SEM will be presented to understand the effect of Ts on the crystal structure and morphology of HfO2 films. Then, the focus will be on the results of cross-sectional SEM imaging coupled with EDS to discuss the effect of Ts on the structural and

Conclusions

HfO2 thin films were grown by sputtering onto Si(1 0 0) varying the Ts from RT to 500 °C. The effect of Ts on the growth, crystal structure, surface morphology and surface/interface structure and composition of the deposited HfO2 films was studied using GIXRD, HR-SEM, and EDS measurements. The results indicate that the effect of Ts is remarkable on the growth, surface and interface structure, morphology and chemical composition of the HfO2 films. HfO2 films grown at Ts < 200 °C were amorphous while

Acknowledgements

One of us (CVR) acknowledges with pleasure the support of UTEP-URI grant. A portion of the research was performed using EMSL, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research located at Pacific Northwest National Laboratory. One of us (BA) sincerely thanks the support of Louis Stokes Alliances for Minority Participation Bridge to the Doctorate (LSAMP BD) program.

References (31)

  • M. Balog et al.

    Thin Solid Films

    (1977)
  • L. Pereira et al.

    Mater. Sci. Eng. B

    (2004)
  • J.C. Kim et al.

    Thin Solid Films

    (2009)
  • J. Robertson

    Solid State Electron.

    (2005)
  • T.-H. Moon et al.

    Micro Elec. Eng.

    (2006)
  • S. Hayashi et al.

    Appl. Surf. Sci.

    (2003)
  • R. Tan et al.

    Appl. Surf. Sci.

    (2004)
  • L.-p Feng et al.

    Vacuum

    (2009)
  • M.T. Bohr et al.

    IEEE Spectr.

    (2007)
  • G.D. Wilk et al.

    J. Appl. Phys.

    (2001)
  • D.G. Schlom et al.

    RS Bull.

    (2002)
  • B.H. Lee et al.

    IEDM

    (1999)
  • H. Yi Wang et al.

    Appl. Phys. Lett.

    (2009)
  • Wang et al.

    Appl. Phys. Lett.

    (2009)
  • K. Cherkaoui et al.

    J. Appl. Phys.

    (2008)
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