Metallization-Induced Oxygen Deficiency of γ-Al2O3 Layers - The

Article pubs.acs.org/JPCC

Metallization-Induced Oxygen Deficiency of γ‑Al2O3 Layers Elena O. Filatova,*,† Aleksei S. Konashuk,† Franz Schaefers,‡ and Valeri V. Afanas’ev§ †

Institute of Physics, St-Petersburg State University, Ulyanovskaya Str. 1, Peterhof, 198504 St. Petersburg, Russia Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Albert Einstein Str. 15, 12489 Berlin, Germany § Department of Physics and Astronomy, University of Leuven, Celestijnenlaan 200D, Belgium ‡

ABSTRACT: γ-Al2O3 layers fabricated by annealing-induced crystallization of ALDgrown amorphous (a-) Al2O3 films were studied using near-edge-X-ray absorption fine structure, soft X-ray reflection spectroscopy, photoelectron spectroscopy, internal photoemission spectroscopy, and X-ray diffraction method. Despite the crystallization at high (1100 °C) temperature expected to deliver a thermodynamically stable Al2O3 phase, the development of oxygen deficiency upon roomtemperature deposition of TiN and TaN electrodes was established. We ascribe this effect to oxygen scavenging by chemically active metals such as Ti and Ta during electrode sputtering. As a result, a high density of gap states is generated in the insulating oxide near the metal surface, which may impair insulating properties of γAl2O3.

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INTRODUCTION

Thanks to the well-known chemical stability and superior electronic properties, (poly)crystalline γ-Al2O3 layers offer an advantageous alternative to the amorphous (a-) alumina films widely applied in a variety of electron devices. For example, the increase in the conduction band (CB) offset at interfaces with semiconductors upon alumina crystallization2,3 allows for better gate insulation of wide-bandgap channel materials such as GaN4 and SiC.5,6 In particular, polycrystalline γ-Al2O3 films obtained by annealing-induced crystallization of the atomiclayer-deposited (ALD) alumina2,7 are seen as superior intergate insulators in charge trapping (flash) memory cells,8 eventually allowing for 3D integration, improving the cell performance9,10 as well as promising a high-temperature operation;11 however, retention properties of the Al2O3-insulated memory cells appear to critically depend on the oxygen deficiency developed during postdeposition anneal (PDA), mandating application of an oxygen-containing ambient.12 In the present work, we will demonstrate that, contrary to the intuitive expectation, γ-Al2O3 layers obtained by high-temperature crystallization annealing may develop oxygen deficiency upon the later processing steps carried out at significantly lower temperature. In particular, the X-ray absorption (XAS) experiments reveal that even in the initially stoichiometric γAl2O3 the room-temperature plasma-enhanced deposition of TiN or TaN electrodes leads to the formation of an O-deficient region in the oxide and gap states, which may cause electron leakage current. Thus, to attain the desirable improvement of the insulating properties by crystallizing the ALD-grown alumina into the γ-phase, one also would need to optimize the metallization scheme to avoid the oxygen loss. © 2016 American Chemical Society

EXPERIMENTAL METHODS

The studied 12 nm thick Al2O3 films were grown by atomic layer deposition (ALD) using trimethylaluminum Al(CH3)3 and water precursors at 300 °C on the IMEC-cleaned 300 mm n-type Si(100) wafers. Next, the samples were annealed in O2 atmosphere at 1100 °C for 60 s to ensure crystallization in γphase without oxygen loss. Metallization was done by physical vapor deposition (PVD) of 10 nm thick TiN or TaN layers on the whole Al2O3 film area. Some of the samples were subsequently annealed in a N2 atmosphere at 850 °C for 60 s, which corresponds to the typical thermal budget of flash cell processing. Finally, the metal layers were lithographically patterned in 1 mm2 area capacitors using HBr/Cl plasma. The distance between the electrodes was ∼4 mm, which exceeds the horizontal size of the light spot in the X-ray absorption (XAS) or X-ray photoelectron spectroscopy (XPS) analysis described later. No residual Cl species were detected by XPS on the surface of Al2O3 films, which may be considered as evidence of absence of plasma damage of Al2O3 films. The absence of the contribution stemming from the oxidized surfaces of TiN or TaN electrodes was established by the XAS in the vicinity of the NK-absorption edge as well as N 1sphotoelectron line in the XPS data. As the reference samples, unmetallized γ-Al2O3/Si and a-Al2O3/Si (amorphous) structures were also fabricated. The studied samples will further be referred to as follows: 1: a-Al2O3/Si, 2: γ-Al2O3/Si, 3: TiN(asdeposited)/γ-Al2O3/Si, 4: TaN(as-deposited)/γ-Al2O3/Si, 5: TiN(annealed)/γ-Al2O3/Si, and 6: TaN(annealed)/γ-Al2O3/Si.

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Received: February 8, 2016 Revised: March 16, 2016 Published: April 7, 2016 8979

DOI: 10.1021/acs.jpcc.6b01352 J. Phys. Chem. C 2016, 120, 8979−8985

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The Journal of Physical Chemistry C

metal electrode into the insulating film. More details of these measurements can be found elsewhere.18

The structure and composition of the Al2O3 layers were analyzed using the near-edge X-ray absorption fine structure (NEXAFS), soft X-ray reflectivity spectroscopy (SXRS), and XPS techniques using the BESSY II synchrotron light source at the Helmholtz-Zentrum Berlin. The SXRS measurements were performed using s-polarized synchrotron radiation at the reflectometer setup mounted on the optics beamline (D-081B2). It was possible to translate the sample within the 20 mm in the direction normal to the incident beam during the experiment without vacuum break, allowing one to take spectra at different points of the sample surface outside the metal electrode. All reflection spectra were measured at grazing incidence angle 2° because the studied Al2O3 films were thick enough to exclude the influence of the Si substrate. Then, the absorption spectra were calculated from measured reflection spectra using the Kramers−Kronig relations as described in the previous publications.13,14 The NEXAFS measurements using differently polarized synchrotron radiation were carried out at the polarimeter station at UE56/2 PGM beamline by monitoring the total electron yield from the samples in a current mode. On both reflectometer and polarimeter stations GaAsP diodes were used as X-ray detectors with a Keithley 617 electrometer as a current meter. The absolute energy calibration was carried out by measuring the energies of the reference N2 lines as well as of the absorption edges for beryllium and iron absorption filters in the first and higher orders of diffraction. The attained energy resolution was better than E/ΔE = 3000, with the accuracy of the energy scale of ∼10 meV. The relative intensities of NEXAFS O K-absorption spectra have been normalized to the continuum jump at the photon energy of 566 eV after subtraction of a background linearly extrapolated from the linear region below the absorption onset. Such normalization provides about the same total oscillator strength for O Kabsorption spectra over the photon energy range of 525−566 eV, which is consistent with a general idea of oscillator strength distribution for the atomic X-ray absorption.15 The XPS measurements were performed at RGL-station on the Russian-German beamline. The XPS spectra were taken at the excitation photon energy of 700 eV using a hemispherical electron energy analyzer (Specs Phoibos 1500). All photoemission spectra were collected with the combined analyzer and monochromator energy resolution better than 430 meV. The binding energy scale was referenced to the value of Au 4f7/2 photoelectron peak position (83.95 eV).16,17 The charge neutralization system (flood electron gun) was used to neutralize charging during X-ray exposure of the insulating Al2O3 layer known to give rise to significant artifacts. In addition to synchrotron measurements, the crystal structure of Al2O3 films was characterized by X-ray diffraction (XRD). XRD pattern was obtained in the asymmetric geometry of a grazing incidence (grazing incidence diffraction (GID)). The measurements were carried out using Cu Kα1 (0.154056 nm) emission line with the angular resolution of the diffractometer of 0.03°. Finally, to evaluate the possible electrical impact of the gap states induced in γ-Al2O3 by metallization and anneal, we determined the energy barrier for electrons between the Fermi level of the metal electrode (TiN or TaN) and the bottom of the oxide CB using internal photoemission (IPE) of electron using the capacitors defined by lithography on the same wafers. In these experiments the barrier height is determined as the minimal photon energy needed to inject an electron from the

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RESULTS AND DISCUSSION Aluminum L2,3 X-ray Absorption. The Al L2,3-absorption spectra of the studied samples calculated from the reflection spectra measured outside the TiN/TaN electrodes and thereby providing the information about the evolution of the Al2O3 films are shown in Figure 1. All spectra exhibit a pronounced

Figure 1. Al L2,3-absorption spectra of the Al2O3 films calculated from the reflection spectra: 1: a-Al2O3/Si, 2: γ-Al2O3/Si, 3: TiN(asdeposited)/γ-Al2O3/Si, 4: TaN(as-deposited)/γ-Al2O3/Si, 5: TiN(annealed)/γ-Al2O3/Si, and 6: TaN(annealed)/γ-Al2O3/Si.

fine structure (the main features are labeled a−d), which is determined by electron transitions from 2p states of Al to the unoccupied molecular orbitals (MOs) of the CB. In different crystalline modifications of alumina (except α-Al2O3 (corundum)) Al atoms occupy both tetrahedrally [AlO4] and octahedrally [AlO6] coordinated sites in different proportion. As a result, each alumina phase exhibits specific energy splitting ΔE between features a and b, reflecting differences in the effective charge on the Al atom. In its turn, the ratio between the intensities of a and b peaks corresponds to the abundance of the tetrahedrally and octahedrally coordinated Al atoms (predominance of AlO6-octahedra leads to an increase in the ratio Ia/Ib).19,20 Analysis of the Al L2,3-absorption spectra reveals that the energy splitting ΔEa‑b = 1.6 eV and the intensity ratio Ia/Ib ≈ 0.8 (with the accuracy limit of ∼0.01 as determined by the uncertainties of inferring the intensities and positions of the 8980

DOI: 10.1021/acs.jpcc.6b01352 J. Phys. Chem. C 2016, 120, 8979−8985

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The Journal of Physical Chemistry C peaks a and b) in all annealed samples (2−6), which correspond to the γ-Al2O3 phase.21,22 The values of the splitting ΔEa‑b = 2.8 eV and the intensity ratio Ia/Ib = 0.61 obtained for the sample 1 [a-Al2O3/Si] correlate well with the characteristics for amorphous alumina20,22,23 and correspond to the predominantly tetrahedral coordination of Al cations. The absence of the fine structure in the band c in the spectrum of aAl2O3 indicates the lack of long-range order in the structure of this film.21,24 It is clear then that the 1100 °C anneal transforms the ALD-grown a-Al2O3 film into a polycrystalline γ-phase. Subsequent deposition of metal electrodes, its composition, and 850 °C annealing do not affect the intensity ratio Ia/Ib in any measurable way, indicating that abundance of the tetrahedrally and octahedrally Al sites remains unchanged. Nevertheless, a noticeable lowering of the overall contrast in the spectrum for the sample 6 [TaN(annealed)/γ-Al2O3/Si] accompanied by a minor decrease in the intensity ratio Ia/Ib to 0.73 (from 0.8) is worth mentioning. Oxygen K X-ray Absorption. Figure 2 shows the O Kabsorption spectra of the previously mentioned samples.

octahedra (or tetrahedra) and have a stronger overlap with 2p orbitals of the neighboring O atoms. As a result, the O K-nearedge structure is very sensitive to changes in the local bonding environment of Al cations. The presence in the main band a double structure a′-a originates from the crystal-field splitting of the Al 3d states into t2g(e) and eg(t2) electronic states of AlO6 (AlO4) structural units. Analysis of the O K-absorption spectra of crystallized samples (2−6) reveals that the spectra agree well with the known spectrum of γ-Al2O3.4,19,22 By contrast, the spectrum of sample 1 [a-Al2O3/Si] shows several distinct differences: (i) the low-energy shift of the O K -absorption edge position versus γAl2O3 (∼0.4 eV) and (ii) the fusion of two features b′-b in a single broad band. In their turn, these changes agree well with the known O K-absorption spectra of amorphous Al2O3.22,23,27 The O K-absorption spectra of the γ-Al2O3 films calculated from the reflection spectra of the samples without (2) or with metallization-annealing processing (5,6) are compared in the Figure 3. The data reveal a substantial decrease in the peak

Figure 3. O K-absorption spectra of the γ-Al2O3 films calculated from the reflection spectra for 2: γ-Al2O3/Si, 5: TiN(annealed)/γ-Al2O3/Si, and 6: TaN(annealed)/γ-Al2O3/Si.

intensity a in the spectra of γ-Al2O3 films subjected to metallization as compared with the pristine γ-Al2O3 film also obtained by PDA. It is known28,29 that the intensity of the peak a in the O K-absorption spectrum reflects the concentration of oxygen in the film. The decrease in this peak points toward the loss of oxygen in the metallization process. Furthermore, the pre-edge region of the O K-absorption spectra exhibits the additional feature β only in the spectra of the metallized films both before and after annealing the TiN or TaN electrodes (cf. Figure 2). The anticorrelation between the intensities of the features β and a in the O K absorption spectra allows one to associate the features β with oxygen deficiency28,29 of the γAl2O3 layer presumably due to oxygen scavenging by active metals such as Ti and Ta during electrode sputtering. In addition, the O K absorption spectrum of sample 6 [TaN(annealed)/γ-Al2O3/Si] exhibits broadening of the main absorption band and a low-energy shift of the absorption edge by ∼0.3 eV (cf. inset in Figure 3). To exclude the possibility that the feature β is due to possible plasma damage of γ-Al2O3 layer during plasma etching of TiN/ TaN electrodes, we have applied a wet etching in H2O2 to sample 6 [TiN(as-deposited)/γ-Al2O3/Si] and measured the O K-absorption spectrum in the bare area of Al2O3 film. The corresponding spectrum is shown in Figure 4 together with the spectrum measured between TiN electrodes before H2O2 etching. One can see that the feature β is still present in the O K-absorption spectrum after H2O2 etching. Because the bare

Figure 2. O K-absorption spectra of the Al2O3 films calculated from the reflection spectra: 1: a-Al2O3/Si, 2: γ-Al2O3/Si, 3: TiN(asdeposited)/γ-Al2O3/Si, 4: TaN(as-deposited)/γ-Al2O3/Si, 5: TiN(annealed)/γ-Al2O3/Si, and 6: TaN(annealed)/γ-Al2O3/Si.

According to the calculation of the partial densities of states (PDOS),25,26 the O K-absorption spectrum of alumina originates from transitions to the unoccupied states of the conduction band formed by 2p states of O mixed with Al t2 (t2g) and Al e (eg) states (main broad band a′-a) and with Al 3s-, Al 3p-, and Al 3d- states (broad features b′-b, respectively). It is known that in the octahedral (or tetrahedral) complexes the metal eg(t2) orbitals are directed toward the corners of the 8981

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Al2O3/Si] as in sample 6 [TaN(annealed)/γ-Al2O3/Si] (cf. Figure 5d). This would be the indication that the electron structure of γ-Al2O3 is generally preserved after metallizationanneal processing. Nevertheless, we refrain from extracting the bandgap width from the O 2p loss band because the need to deconvolve the “primary” O 1s zero-loss peak to recover the single-electron loss function faces a number of problems that impair the accuracy of this technique.30 It is worth adding here that frequently applied linear fit of the loss function is inconsistent with physics of electron energy loss through excitation of band-to-band electron transitions in a solid31,32 and cannot be considered as a valid procedure of the absolute loss threshold determination. This kind of analysis is beyond the scope of this study. Another important result delivered by the XPS analysis concerns the contamination levels of the pristine and premetallized γ-Al2O3 surfaces: The intensities of both the contaminant-related O 1s component shown in Figure 5b,c and that of the C 1s peak (not shown) indicate the same contamination level of the samples prior to and after metallization. This observation indicates that the pre-edge peak β in the O K absorption spectra (cf. Figures 2 and 3) are not related to the surface-contaminating molecules (CO, CO2, hydrocarbons). At the same time, observation of the same feature β for two different electrode materials (TiN, TaN) suggests that it pertains to some (intrinsic) electron state in the γ-Al2O3 layer. Besides, it is worth mentioning that the XRD scans indicate that the γ-Al2O3 crystallites in sample 6 [TaN(annealed)/γ-Al2O3/Si] is lacking preferential orientation as opposed to the other γ-Al2O3 layers studied in this work. Furthermore, only for this sample was the shape of the O Kabsorption spectrum sensitive to the method used, that is, reflection or the total electron yield measurements. Moreover, for the total electron yield method applied to sample 6 a significant point-to-point variability of the spectrum was also found. Taking together with the previously mentioned broadening of the O K absorption band and the slight shift (∼0.2 eV) of the Al 2p binding energy, these observations indicate that modification of the surface structure of the γ-Al2O3 layer does occur upon deposition-annealing of the TaN electrode. In its turn, this interaction can be correlated with maximal intensity of the pre-edge β peak in the O K spectra (Figures 2 and 3) pointing toward the relationship of it to a defect state in the polycrystalline γ-Al2O3 modified by metallization. Oxygen K NEXAFS Analysis. Let us turn to more detailed analysis of the pre-edge peak β in the O K-absorption spectra of γ-Al2O3. Figure 6 shows the pre-edge region of O K-absorption spectra for samples 5 [TiN(annealed)/γ-Al2O3/Si] and 6 [TaN(annealed)/γ-Al2O3/Si]. The O K-absorption spectra shown in Figure 6 were measured by the total electron yield method using both left and right elliptically polarized radiation. As follows from the Figure 6, the energy position of the peak β does not depend on the material of the electrode and as a consequence on the crystalline phase of γ-Al2O3 (monocrystalline or polycrystalline γ-Al2O3). Remarkably, while the intensity and the shape of the main O K-band (a′-a) and the features b′-b in both samples are identical for left and right elliptical light polarization, the intensity of the pre-edge peak β appears to be sensitive to the direction of elliptical polarization. For both TiN(annealed)/γAl2O3/Si and TaN(annealed)/γ-Al2O3/Si stacks the intensity of peak β is greater for left rotation of polarization vector. The

Figure 4. O K-absorption spectra of the γ-Al2O3 film in TiN(asdeposited)/γ-Al2O3/Si outside the TiN electrodes (curve 3) and in the bare area of Al2O3 film after a wet etching of TiN electrode in H2O2 (curve 3*). Both curves are measured by the total electron yield method. The spectra are normalized to the same continuum jump after subtraction of sloping background.

area of Al2O3 film obviously was not subjected to plasma etching, one can associate the feature β with oxygen deficiency of the γ-Al2O3 layer, which is due to oxygen scavenging by metal electrode. Aluminum 2p and Oxygen 1s X-ray Photoelectron Spectra. The Al 2p XPS spectra shown in Figure 5a suggest a

Figure 5. Al 2p and O 1s photoelectron spectra measured at excitation energy of 700 eV and normal emission angle outside the electrode. (a) Al 2p spectra of 2: γ-Al2O3/Si, 5: TiN(annealed)/γ-Al2O3/Si, and 6: TaN(annealed)/γ-Al2O3/Si. (b,c) O 1s spectra taken from γ-Al2O3 surfaces in samples 6 and 2, respectively. (d) Scaled area of their loss spectra.

slight shift (∼0.2 eV) of the binding energy in the sample with TaN metallization processing. Surprisingly, however, the binding energy of Al 2p level in sample 6 [TaN(annealed)/γAl2O3/Si] is the same as in a-Al2O3 film,22 while the Al L2,3- and O K-absorption spectra (Figures 1 and 2) demonstrate the γphase of Al2O3 in this sample. The O 1s photoelectron spectra shown in Figure 5b,c indicate that the electron energy loss band corresponding to electron excitation across the band gap of γAl2O3 has the same shape in the unmetallized sample 2 [γ8982

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Figure 6. Pre-edge region of O K-absorption spectra of γ-Al2O3 films in samples 5 [TiN(annealed)/γ-Al2O3/Si] and 6 [TaN(annealed)/γ-Al2O3/Si]. The spectra were measured by total electron yield using right and left elliptically polarized radiation. The incidence angle relative to the sample surface was equal to 30° for sample 5 [TiN(annealed)/γ-Al2O3/Si] and 90° for and for sample 6 [TaN(annealed)/γ-Al2O3/Si]. The spectra were normalized to the continuum jump after subtraction of sloping background.

Figure 7. Semilogarithmic (a) and Fowler (b) spectral plots of the electron photoemission quantum yield from TaN and TiN electrodes into γAl2O3 layer. The results are shown for the as-deposited metal electrodes and for the samples subjected to the postmetallization anneal (PMA) in N2 at 850 °C. All spectra are measured when applying the same −2 V bias to the metal electrodes. The inset in panel b illustrates the schematics of the observed optically excited electron transitions.

interface with uniform barrier height (which should yield a linear plot in these coordinates), the IPE spectra exhibit significant spread of the barrier values. The latter is consistent with the formation of laterally nonuniform barrier due to trapping of charge in the near-interface oxide layer. Furthermore, if the density of the defects becomes high enough or the Al2O3 thickness decreases, it is possible that defect-to-defect or defect-to-electrode electron transitions will give rise to leakage current. Therefore, we have reason to believe that the gap states associated with metallization-induced O deficiency of γ-Al2O3 revealed in the present study are directly relevant to the electrical degradation of the insulating properties mentioned in the Introduction.

shown spectra were measured at incidence angles of 30° and 90° on samples 5 [TiN(annealed)/γ-Al2O3/Si] and 6 [TaN(annealed)/γ-Al2O3/Si], respectively. The spectra measured at 90° incidence angle on sample 5 [TiN(annealed)/γ-Al2O3/Si] were indistinguishable for two different light polarizations. Although we hypothesize that the exact origin of this anisotropy may be related to the preferential orientation of spin states involved in the X-ray absorption,33 what is more important here is that the energy position of the peak β can accurately be referenced to the onset of electron transitions from the O 1s level to the states of CB of γ-Al2O3: The unoccupied gap states in the oxide are distributed in the energy range of 2 to 3 eV below the bottom edge of the γ-Al2O3 CB.22 TiN/γ-Al2O3 and TaN/γ-Al2O3 Barrier from Internal Electron Photoemission. To evaluate the impact the Odeficiency-related unoccupied states on the insulating performance of thin γ-Al2O3 layers relevant for device application, we compared their energy distribution to the energies of the Fermi levels of the TiN and TaN electrodes measured relative to the same energy of the oxide CB bottom edge. The IPE spectra illustrated in Figure 7 indicate that the electron IPE from TaN is observed in the photon energy range hν > 2.5 eV, while electron emission from TiN requires somewhat higher energies hν > 3 eV. Obviously then, the unoccupied gap states distributed in the energy range 2 to 3 eV below the oxide CB will overlap with the distribution of electron states in the metal electrodes. As a result, electrons can be trapped by these defects, leading to the oxide charging. In fact, the Y1/2−hν (Fowler) plots shown in panel b indicate that instead of an

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CONCLUSIONS

The presented results indicate that a rather unexpected effect influences the γ-Al2O3 layers fabricated by annealing-induced crystallization of ALD-grown a-Al2O3 films: Despite being exposed to a high temperature (1100 °C in our case) during crystallization anneal and therefore attaining thermodynamically stable atomic structure, the room-temperature metal deposition causes the development of oxygen deficiency. We ascribe this effect to the oxygen scavenging by active metals such as Ti and Ta during electrode sputtering. As a result, a high density of gap states is generated in the insulating oxide near the metal surface, which is expected to impair insulating behavior of γ-Al2O3. 8983

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +7 (812) 428 43 52. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was partially supported by SPbSU grant 11.37.656.2013. We gratefully acknowledge assistance from the bilateral Program “Russian-German Laboratory” at HZBBESSY II. We gratefully acknowledge the financial support by Helmholtz Zentrum Berlin (HZB) and also thank HZB for the allocation of synchrotron radiation beamtime. We gratefully acknowledge the Research Centre for X-ray Diffraction Studies at St. Petersburg State University, Russia for the diffraction experiments.

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DOI: 10.1021/acs.jpcc.6b01352 J. Phys. Chem. C 2016, 120, 8979−8985