Elsevier

Thin Solid Films

Volume 467, Issues 1–2, 22 November 2004, Pages 16-27
Thin Solid Films

Nucleation and growth during the atomic layer deposition of W on Al2O3 and Al2O3 on W

https://doi.org/10.1016/j.tsf.2004.02.099Get rights and content

Abstract

Nucleation and growth are critical during the atomic layer deposition (ALD) of ultra thin films and nanolaminates. This study examined the nucleation and growth during tungsten (W) ALD on aluminum oxide (Al2O3) surfaces and Al2O3 ALD on W surfaces using Auger electron spectroscopy (AES). W ALD was performed using alternating exposures of WF6 and Si2H6. Al2O3 ALD was performed using alternating exposures of Al(CH3)3 and H2O. AES signals were measured after each WF6 and Si2H6 exposure during W ALD on Al2O3 and after each Al(CH3)3 and H2O exposure during Al2O3 ALD on W. The AES measurements revealed that 3 WF6/Si2H6 reaction cycles were required to nucleate the W ALD film on Al2O3 surfaces at 473 K. Subsequently, the W ALD film grew linearly at a rate of 2.6–3.5 Å per WF6/Si2H6 reaction cycle. The AES measurements also revealed that only one H2O/Al(CH3)3 cycle was needed to nucleate Al2O3 ALD on W at 450 K. Subsequently, the Al2O3 ALD film grew linearly at the rate of 1.0 Å per Al(CH3)3/H2O reaction cycle. As expected from the W ALD surface chemistry, the W and Si AES signals oscillated dramatically during the sequential WF6 and Si2H6 exposures. Many parameters were varied to determine their effect on the W ALD nucleation period. The WF6 surface reaction was surprisingly insensitive to the Al2O3 substrate temperature and the initial hydroxyl coverage on the Al2O3 surface. These results for the nucleation and growth during W ALD on Al2O3 and Al2O3 ALD on W are relevant to the growth of W/Al2O3 nanolaminates that have potential as X-ray mirrors, thermal barrier coatings and tribological films.

Introduction

Atomic layer deposition (ALD) can provide atomic layer control of thin film growth using sequential, self-limiting surface reactions [1], [2], [3]. ALD has many advantages for thin film growth including exact film thickness and excellent film conformality on high aspect ratio structures [4]. The nucleation of ALD film growth is critical for precise thickness control and maximal smoothness. Long nucleation periods can jeopardize film conformality and lead to unacceptable surface roughness. Nucleation processes are especially critical during the growth of nanolaminates because of their high interfacial density [5].

This study examines the nucleation and growth during W ALD on Al2O3 and Al2O3 ALD on W. Al2O3 ALD growth is accomplished using sequential exposures to trimethylaluminum (TMA) and H2O through the following binary reaction sequence [6], [7], [8]:(A)AlOH*+Al(CH3)3AlOAl(CH3)2*+CH4(B)AlCH3*+H2OAlOH*+CH4where the asterisks designate the surface species. These two reactions are self-limiting and terminate after the consumption of all the reactive surface species. During the (A) reaction, Al(CH3)3 reacts with surface hydroxyl species, AlOH*, and deposits surface AlCH3* species [8]. In the (B) reaction, H2O reacts with surface methyl species, AlCH3*, and rehydroxylates the surface [8]. Application of these reactions in an ABAB… binary sequence yields an Al2O3 film with an Al2O3 ALD growth rate of 1.1 Å/cycle [6], [7]. Al2O3 ALD has been studied extensively and is an ideal ALD system [6], [7], [8].

Single-element W films can also be deposited using ALD techniques [9], [10]. The ALD of single-element films requires a different surface chemistry than the surface chemistry employed for binary compounds like Al2O3. For W ALD, one of the reactants is a sacrificial species that serves as a place holder in the ABAB... binary reaction sequence. This sacrificial species is removed during the subsequent surface reaction. The surface chemistry for W ALD is based on sequential WF6 and Si2H6 exposures and can be written as [9], [10]:(A)WSiHyFz*+WF6WWFx*+H2+SiHaFb(B)WFx*+Si2H6WSiHyFz+H2

During the (A) reaction, WF6 reacts with the sacrificial silicon surface species, WSiHyFz*, and deposits WFx species [9]. In the (B) reaction, Si2H6 strips fluorine from the tungsten surface species, WFx*, and reforms the sacrificial silicon surface species [9]. The reaction stoichiometry is kept undefined because the exact identity of the surface species is not known. Application of these reactions in an ABAB… reaction sequence yields smooth W ALD films with a reported growth rate of ∼2.5 Å/AB cycle [9]. Additional studies have examined the nucleation and growth during W ALD on SiO2 [11] and the kinetics of the WF6 and Si2H6 surface reactions [12].

Nucleation may not occur during the first reaction cycle during ALD [13], [14]. The initial substrate may not have the appropriate surface species to facilitate the ALD growth. Earlier studies of the nucleation and growth of W ALD on SiO2 dramatically illustrate these nucleation difficulties [11]. W ALD on SiO2 required between 8 and 9 AB cycles to nucleate and form one W monolayer on the SiO2 surface [11]. This nucleation period may affect the measurement of the W ALD growth rate. In addition, the number of WF6/Si2H6 reaction cycles required to nucleate the W ALD film may affect the resulting W film roughness.

Understanding nucleation is especially critical when ALD techniques are used to deposit nanolaminates [5]. The high interfacial density in nanolaminates amplifies the affects of the nucleation period. One important nanolaminate is the W/Al2O3 superlattice that may be important for X-ray mirrors [15]. The W/Al2O3 nanolaminates may also be useful as low thermal conductivity thermal barrier coatings [16] or tribological films [17]. W ALD nucleation on Al2O3 nanolayers and Al2O3 ALD nucleation on W nanolayers occurs repeatedly when fabricating the W/Al2O3 nanolaminate. Consequently, roughness introduced by W ALD nucleation on Al2O3 or Al2O3 ALD nucleation on W may accumulate and propagate through the nanolaminate structure.

To understand nucleation and growth during W ALD on Al2O3 and Al2O3 ALD on W, Auger electron spectroscopy (AES) measurements were performed after each reactant exposure. For W ALD on Al2O3, growth of W AES signals and reduction of O AES signals versus the sequential WF6 and Si2H6 exposure were used to determine the nucleation period and the subsequent W ALD growth rate. For Al2O3 ALD on W, the reduction of W AES signals versus the sequential Al(CH3)3 and H2O exposures was used to determine the nucleation period and the subsequent Al2O3 ALD growth rate.

Section snippets

Vacuum chamber, sample preparation and reactant exposures

The nucleation and growth chemistry were monitored using Auger electron spectroscopy (AES) in an ultra high vacuum (UHV) apparatus. This instrument has been described earlier in various publications [11], [12], [18], [19]. This UHV chamber was equipped with an internal high pressure reaction cell for reactant exposures at pressures as high as 15 Torr [19]. The chamber also contained an internal capillary array doser for reactant exposures at lower pressures. The internal high pressure reaction

AES during W ALD on Al2O3 surfaces

Fig. 1 shows the Auger electron spectroscopy (AES) results during W ALD on Al2O3 at 473 K. The top spectrum shows the AES spectrum from the initial Al2O3 substrate. This AES spectrum shows only two aluminum features at 35 and 51 eV and the dominant oxygen feature at 503 eV. The initial Al2O3 surface was hydroxylated by stopping the initial Al2O3 ALD after the H2O reaction as described by Eq. (2). The WF6 was the first reactant exposure during W ALD. During the 4th AB cycle after the WF6

W ALD on Al2O3 from W AES intensities

Knowledge of the electron attenuation lengths through the various materials at different electron energies leads to quantitative information from the AES spectra. Nucleation and growth information is obtained using the AES signals from both the growing ALD film and the underlying substrate. When tungsten is deposited conformally on a substrate, the intensity of the tungsten AES signal, IW, is a function of the thickness, d, of the tungsten layer as follows:IW(d)=IW0[1−exp(d/λ1)]In this

Conclusions

The nucleation and growth during W ALD on Al2O3 and Al2O3 ALD on W were examined using Auger electron spectroscopy (AES) techniques. During W ALD, AES signals were measured after each WF6 and Si2H6 exposure in the ABAB... reaction sequence. The AES measurements revealed that 3 AB cycles were required to nucleate the W film on Al2O3 surfaces at 473 K using reactant exposures of 130 x 103 L for WF6 and 30×103 L for Si2H6. Subsequently, the W film grew linearly at a rate of 2.6–3.5 Å/cycle with

Acknowledgements

This work was supported by a the Air Force Office of Scientific Research. Additional support was obtained from Seagate Technology. The authors thank Dr. J.W. Elam for the useful discussions and S.J. Ferro for the initial work on the fabrications of W/Al2O3 nanolaminates.

References (37)

  • T. Suntola

    Thin Solid Films

    (1992)
  • A.W. Ott et al.

    Thin Solid Films

    (1997)
  • A.W. Ott et al.

    Appl. Surf. Sci.

    (1996)
  • A.C. Dillon et al.

    Surf. Sci.

    (1995)
  • J.W. Klaus et al.

    Thin Solid Films

    (2000)
  • J.W. Klaus et al.

    Appl. Surf. Sci.

    (2000)
  • J.W. Elam et al.

    Thin Solid Films

    (2001)
  • J.W. Elam et al.

    Surf. Sci.

    (2001)
  • W.F.A. Besling et al.

    J. Non-Cryst. Solids

    (2002)
  • P.C. Yashar et al.

    Vacuum

    (1999)
  • C.E. Nelson et al.

    Surf. Sci.

    (1998)
  • D.C. Jackson et al.

    Surf. Sci.

    (1973)
  • H. Tokutaka et al.

    Surf. Sci.

    (1983)
  • E. Umbach et al.

    Surf. Sci.

    (1983)
  • J.W. Elam et al.

    Thin Solid Films

    (2002)
  • R. Matero et al.

    Thin Solid Films

    (2000)
  • S.M. George et al.

    J. Phys. Chem.

    (1996)
  • M. Ritala et al.
  • Cited by (60)

    • Influence of the surface roughness of the bottom electrode on the resistive-switching characteristics of Al/Al<inf>2</inf>O<inf>3</inf>/Al and Al/Al<inf>2</inf>O<inf>3</inf>/W structures fabricated on glass at 300 °c

      2014, Microelectronics Reliability
      Citation Excerpt :

      Otherwise, incomplete nucleation of Al2O3 would produce only the LRS in the I–V data since both electrodes (TE and BE) would be in direct contact. Additionally, even though tungsten is a highly inert metal as compared to aluminum, it has been reported that Al2O3 does not have severe nucleation problems when deposited on tungsten since complete nucleation requires only one full ALD cycle [13,14]. Therefore, the physical and/or chemical origin of these instabilities is important in order to engineer processing solutions for this problem and thus, obtain reproducible I–V cycles during ReRAM operation.

    • Platinum thin films with good thermal and chemical stability fabricated by inductively coupled plasma-enhanced atomic layer deposition at low temperatures

      2014, Thin Solid Films
      Citation Excerpt :

      A self-limiting growth mechanism facilitates the growth of conformal thin films over large areas with an accurate thickness in each deposition cycle [3]. Various dielectric materials have been deposited using these technologies, including Al2O3 [3–5], polycrystalline luminescent ZnS:Mn [6], TiO2 [7, 8], TaNx [9, 10], ZrO2 [11], and ZnO [12]. Many types of metals have also been deposited with these technologies, including Ru [13, 14], Pt [15–18], and Cu [19].

    • Energy-enhanced atomic layer deposition for more process and precursor versatility

      2013, Coordination Chemistry Reviews
      Citation Excerpt :

      AlCl3(s) + Al(OEt)3(vap) → Al2O3(s) + 3EtCl(g)↑ Other interesting ALD chemistries, illustrating the wealth of viable ALD processes, include the deposition of metallic tungsten from WF6 and Si2H6 [28–30], and the syntheses of GeSb, GeTe and Sb2Te3 from the respective metal chloride, GeCl2·C4H8O2 (C4H8O2 = 1,4-dioxane) and SbCl3, with trialkylsilyl compounds, E(SiEt3)n (E = Sb, n = 3; E = Te, n = 2) [31–33]. These examples both demonstrate how halo(alkyl)silane elimination reactions, driven by the formation of strong silicon–halogen bonds, can be exploited to give an efficient ALD reaction (Eq. (2)).

    View all citing articles on Scopus
    View full text