Nature of Hydrophilic Aluminum Fluoride and Oxyaluminum Fluoride

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Nature of Hydrophilic Aluminum Fluoride and Oxyaluminum Fluoride Surfaces Resulting from XeF2 Treatment of Al and Al2O3 K. Roodenko,*,† M. D. Halls,‡ Y. Gogte,† O. Seitz,† J.-F. Veyan,† and Y. J. Chabal† †

Laboratory for Surface and Nanostructure Modification, Department of Materials Science & Engineering, University of Texas at. Dallas, 800 West Campbell Road, Mail Station RL10, Richardson, Texas 75080, United States ‡ Materials Design Inc, 11417 West Bernardo Road, San Diego, California 92127, United States

bS Supporting Information ABSTRACT: XeF2 treatment of aluminum and alumina surfaces is known to produce hydrophilic surfaces. There is however poor knowledge of the chemical nature of these surfaces. Using infrared absorption and X-ray photoelectron spectroscopy, the formation of highly hydrophilic AlF3 and AlOxFy surface layers is identified upon XeF2 exposure, along with strongly bound H2O and other related surface species formed by interactions with trace H2O under typical vacuum conditions (≈10 4 Torr). Surfaces resulting from XeF2 etching of oxide-free aluminum covered by a sacrificial Si layer have a strong affinity for H2O, with a contact-angle of ca. 5 10°. First-principles simulations offer new insight into details of the AlFx surface structure, based on the surface IR characterization by providing reliable assignments for associated AlFx 3 3 3 H2O infrared bands and showing that fluorine is strongly bound to Al, preventing further Al oxidation. The formation of hydrophilic AlF3 surface layers upon fluorine-based etching may pose a fundamental limitation for the use of Al in microelectromechanical (MEMs) applications, precluding the release of low-stiction, low-capillary force components.

’ INTRODUCTION Xenon difluoride (XeF2) is frequently used as a dry etchant for release of microelectromechanical (MEMs) devices. When a Si, Mo, or Ge layer is exposed to XeF2 vapor, rapid isotropic removal of the sacrificial film is observed.1 One of the benefits of XeF2 etching is its chemical selectivity, being largely inert against SiO2, Si3N4, polymers, and many metals and dielectrics. Etching of the sacrificial material (e.g., Mo or Si) occurs by the successive fluorination of the top-layer and accessible subsurface atomic sites, leading to the release of labile highly fluorinated molecular species. The final step in XeF2 etching involves reaction at the interface between the sacrificial film and device substrate which leads to fluorination of the underlying surface. For MEMs applications, controlling the chemical nature of the released surfaces is of critical importance in order to determine the limitations of etch/stop material combinations. Therefore, the chemical characterization of surfaces fluorinated by interaction with XeF2 is of importance for the development of microelectronic and MEMs devices. Due to their widespread application as conductive and dielectric layers, several initial studies on the effect of XeF2 etching on Al and Al2O3 surfaces were reported.2,3 Upon fluorination of Al2O3 and Al surfaces by reaction with XeF2, strong hydrophilicity and surface adhesion were observed. These properties were attributed to the formation of AlF3 on the surface, which is known to be an extraordinarily strong Lewis acid. Due to their strength as Lewis acids, AlF3 substrates of different preparations have received widespread attention in the catalysis community for halogen exchange reactions and the production of hydrofluorocarbons.4 6 r 2011 American Chemical Society

Previous studies have examined the chemical states of XeF2 treated Al and catalytic AlF3 solids using XPS,2,7 10 providing partial information about the chemistry of the AlFx surfaces. Recently, synthesis of high surface area AlF3 via a plasma-assisted fluorination reaction with nitrogen trifluoride was reported by Delattre et al.11 Given the importance of AlF3 in the catalysis community, first-principles calculations of pristine α- and β-AlF3 surfaces have been also been reported;12 14 however, no attempt has been made to calculate characteristics that would be useful in supporting efforts to use vibrational spectroscopy to directly probe the AlFx surface species. The present work employs in situ infrared and ex situ X-ray photoelectron spectroscopy to characterize the surface and interface structures of aluminum fluoride and oxyaluminum fluoride formed upon etching of a Si overlayer from an otherwise oxidefree Al substrate using XeF2 vapor and, for comparison, by the direct interaction of XeF2 with Al2O3. Additionally, the interaction of the XeF2-treated aluminum substrates with H2O is investigated, as it directly pertains to MEMs fabrication and has important implications for the surface adhesion on the mesoand nanoscale.2 The experimental characterization is complemented by first-principles simulations of possible surface structure vibrational signatures. Comparison of the calculated wavenumbers and IR spectroscopic observations reveal details of the bonding configuration at the etched AlFx surfaces and clear Received: August 15, 2011 Revised: September 12, 2011 Published: October 06, 2011 21351

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evidence for adsorbed H2O due to trace water contamination in the reactor.

’ EXPERIMENTAL SECTION Preparation of Al and Al2O3 Substrates. Polycrystalline Al films were deposited on glass substrate by DC sputtering at room temperature. The nominal thickness of the Al layer was 200 nm. Polycrystalline amorphous Si films were deposited on top of the Al films by DC sputtering at room temperature, with no vacuum break between the deposition of Al and Si layers. The nominal thickness of the Si overlayer was 100 nm. Al2O3 surfaces were formed by exposing unprotected Al films to the ambient conditions (above half a year of storage in atmospheric conditions). Sample Fluorination. To generate XeF2 gas, powdered XeF2 compound (used as received from Pelchem) was transferred into an experimental container under N2 gas atmosphere. Vapor pressure of XeF2 was introduced into the reaction chamber through the system described in detail in ref 15. XeF2 etch of a protective Si layer and subsequent fluorination of the exposed Al surface was conducted in a 1 L volume reactor (base pressure of 10 4 Torr), equipped with KBr windows for in situ infrared (IR) detection of the etched gas-phase species.15,16 AlFx surfaces were formed upon XeF2-assisted etching of the Si sacrificial material from aluminum surfaces. XeF2 was introduced at 1.4 Torr (the XeF2 vapor pressure) for 5 min and then pumped out. The IR spectra of gas-phase reactants (XeF2, SiF4) are recorded in situ. This cycle is repeated until no more SiF4 evolution due to the etching was detected.3 For preparation of AlOxFy surfaces, Al2O3 films were exposed to XeF2 at 1.4 Torr for 5 min. Then, the residual gases were pumped out of the reactor. This cycle was repeated four to five times, until the consumption of all XeF2 due to the sample fluorination and the removal of surface contaminants, thus minimizing contamination during the actual experiments. The consumption of gas phase XeF2 was monitored in situ by IR during the exposure procedure. Surface Characterization. The Fourier transform infrared (FT-IR) spectra were recorded with a Nicolet Nexus 6700 spectrometer using a DTGS detector at 4 cm 1 resolution. X-ray photoelectron spectroscopy (XPS), performed with a Perkin-Elmer PHI 5600 ESCA system, was performed ex situ (i.e., after transfer of the sample in air) to characterize the composition of the thin films. The photoelectrons were excited using monochromatic Al Kα radiation (hν = 1486.6 eV). The spectra were scanned at a 45° using 0.125 eV step size. Deconvolution of the spectra was performed using polynomial background subtraction. Al 2p spectra were deconvoluted into doublets with fwhm and spin orbit splitting of 0.32 eV with the ratio of 0.5. Contact-angle measurements were performed on a standard goniometer, model 200-F, from Ram
e-Hart Instruments, Co. The size of the drop was 2 μL. Computational Methods. The first-principles simulation results described here were computed using density functional theory (DFT) with VASP 5.2,17 within the MedeA simulation environment.18 Calculations were carried out within the generalized gradient approximation (GGA-DFT) using the PBE functional19 along with an all-electron frozen core projector augmented plane wave basis set using an energy cutoff of 400 eV. A k-point mesh with a spacing of 0.3 Å 1 was used to sample the electronic Brillouin zone. The Al/AlF3 model is based

Figure 1. (a) Gas-phase data obtained from the reactor containing Sicoated Al surfaces at the end each XeF2 cycle. Inset: HF-related absorption bands observed after each cycle of XeF2 exposure. (b) IR data obtained from aluminum surfaces after exposures to XeF2. Each spectrum is referenced to the spectrum obtained from the surfaces prior to each XeF2 exposure.

on a 2  2 supercell of a three-layer slab of the (111)-Al surface with 2 AlF3 units at the surface. The fully periodic AlF3/Al model was structurally optimized, following a molecular dynamics simulation run at 300 K using a time step of 2 fs for a duration of 4 ps, to allow the Al/AlF3 interface to adopt a low energy configuration. The resulting AlF3/Al surface model is representative of the Al surface after XeF2 exposure. For further details, see the Supporting Information.

’ RESULTS Fluorination of Al Surfaces. Infrared spectra can provide critical information about the local chemical structure of the resultant surfaces and released gases after XeF2 exposure in typical reactor conditions.15 Figure 1a shows the infrared spectra of gas-phase species inside the reactor obtained at the end of each cycle, during which the Si-protected Al surfaces were exposed to XeF2. After the first two cycles, the IR data indicate that the reaction of XeF2 with Si results in the release of SiF4 gas-phase species from the surface. At the same time, the spectrum in Figure 1b obtained after pumping the remaining gases (referencing to the surface prior XeF2 exposure) shows the surface species remaining on Al after each XeF2 exposure. Removal of Si is complete after the first two cycles, leading to fluorination of Al by the XeF2 gas. The successive exposures 3 and 4 (as marked in Figure 1) indicate that there is no further XeF2 consumption, which is confirmed by the presence of the absorption band doublet due to XeF2 positioned at 550 and 567 cm 1.20 Figure 1b shows the IR spectrum obtained from the surface of the exposed substrates after XeF2 exposure, characterized by the appearance of a broad positive absorption band between 530 and 870 cm 1, with broad peaks positioned at 798 and 735 cm 1. This envelope is indicative of the formation of aluminum fluoride and surface 21352

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Figure 2. IR absorbance spectrum obtained from XeF2-exposed surface of native Al2O3 (after four cycles of XeF2).

oxyaluminum fluoride species, as well as of water-derived surface species (as discussed later), after the second cycle of exposure to XeF2. The appearance of water-derived surface species upon fluorination can only be due to interaction with trace amounts of H2O in the chamber and reflects the high hydrophilicity of the surface. The inset in Figure 1a shows a set of HF-related absorption bands which are present in the gas-phase IR data after each cycle of exposure to XeF2 and provides additional support for the reaction of trace H2O with the fluorinated surface with the associated release of HF. Spectra 3 and 4 in Figure 1b indicate that no reactions occur between XeF2 and Al surfaces upon successive exposures to XeF2, suggesting the Al surface is fully passivated by a AlF3 layer and that a steady state is achieved. Fluorination of Al2O3 Surfaces. To gain insight into the structure of the Al/AlF3 substrate, formed by exposure to XeF2, a reference experiment was performed involving XeF2 treatment of an unprotected oxidized aluminum surface (Al2O3). Al2O3 represents a useful empirical reference for the analysis of the XPS and IR spectra of the fluorinated Al surface. Figure 2 shows the IR spectra obtained from Al2O3 surfaces after four XeF2 exposures, referenced to the spectrum obtained prior to etching. As was already noticed by Hills,21 XeF2 removes hydrocarbon contaminants as seen here by the negative absorption in the CHx stretching and bending regions of the spectrum. Removal of surface OH groups is apparent by the appearance of negative absorption bands at around 3500 and 1650 cm 1. An important observation in the fluorinated Al2O3 surface (F/Al2O3) IR spectrum is the appearance of a band with strong intensity at 768 cm 1. The next section discusses the detailed surface composition of XeF2-fluorinated Al2O3, based on the combined XPS and IR studies. Surface Structure of Fluorinated Al and Al2O3 Surfaces. Figure 3 shows the infrared spectra and XPS Al 2p core-level spectra for fluorinated- aluminum and aluminum oxide surfaces (top and bottom panels, respectively). The XPS spectrum from an untreated Al2O3 surface is also shown for a peak position reference in Figure 3e. For the XeF2 exposed Al surface, the infrared spectrum shows the development of a complex and broad envelope in the range from 530 to 870 cm 1 (Figure 3a). As noted above, the XeF2 exposed Al2O3 surface spectrum is marked by a single welldefined band at 768 cm 1 (Figure 3b). Initial guidance as to the identity of these bands might be gained by looking to previous IR studies. Theoretical calculations and experimental data of vibrational modes of solid crystalline and amorphous AlF3 were reported in ref 22. Table 1 summarizes experimental AlF and AlOxFy related absorption bands. Considering the wavenumbers presented there show that the oxygen containing AlFx species have a characteristic frequency comparable to the band 768 cm 1

Figure 3. (a, b) Infrared spectra in the 500 900 cm 1 spectral range showing AlOxFy absorption bands for XeF2-fluorinated Al and Al2O3 surfaces. (c e) XPS data obtained from (c) XeF2 fluorinated aluminum, (d) XeF2 fluorinated Al2O3, and (e) untreated Al2O3 surface, which is given for peak assignment reference.

Table 1. Experimental Wavenumbers for AlOxFy-Related Infrared Absorption Bands wavenumber (cm 1)

assignment

ref

965

AlF3 (gas-phase)

24

995

Al2F3 (gas-phase)

24

793 1147

Al F (gas-phase) ν(Al O) in OAlF (argon matrix)

24 23, 26

740

ν(Al F) in OAlF (argon matrix)

23

585

AlF6 x(OH)x3‑ (KBr pellet)

27

570

AlF63‑(KBr pellet)

27

660 645

AlF3(KBr pellet)

27

640 666

AlF3 (CsI, KBr pellets)

22

667

AlF3 (KBr pellet)

7

650 750

AlF3 (CsI pellet)

28

in fluorinated Al2O3 and toward the higher energy modes of the AlF3 range. In experiments in an argon matrix, Schn€ockel23 showed that OAlF species result in absorption bands at 1147 and 740 cm 1. AlFx-related argon matrix experiments24 showed an absorption band at 776 cm 1. Also here, the higher-frequency absorption bands are interpreted as ν(Al O) while the lower lying bands are interpreted as ν(Al F). Krahl et al.25 have measured AlF3 samples with different degrees of crystallinity. The amorphous AlF3 samples showed very broad absorption bands, generally between 600 and 750 cm 1.25 XPS spectra are complementary to infrared spectra, allowing the identification of the chemical state of the species in the XeF2 treated Al and Al2O3 surfaces. Figure 3c e shows XPS data obtained from fluorinated aluminum, fluorinated Al2O3, and 21353

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Figure 4. Infrared spectrum for the fluorinated aluminum surface after exposure to D2O vapor, referenced to the spectrum obtained after XeF2 etching. Evidence for the adsorption of H2O from the trace water species in the reactor on the surface is clear. The positive absorption bands are due to D2O exchanged with the H2O (negative absorption bands).

untreated Al2O3 surface, for comparison. Metallic aluminum is indicated on each surface by the deconvoluted peak positioned at 72.6 eV. This peak serves as the reference for calibration of any charging effects during the XPS data acquisitions. On the fluorinated Al surface, the deconvolution requires the addition of a peak positioned at 73.2 eV due to suboxides (e.g., AlO OH species, etc.). Comparing the fluorinated and native Al2O3 surfaces, a peak is observed positioned at 75.2 eV (a typical chemical shift of Al in Al2O3-related oxidation state). This peak is absent from the XeF2-treated Al surface. A higher-oxidation state of Al is signified by a peak at 76.4 eV which appears on both the fluorinated Al and Al2O3 surfaces. This peak is due to a common oxyaluminum fluoride species on the surfaces of Al/AlF3 and F/Al2O3; the same species that gives rise to the band at 768 cm 1 in the IR for the fluorinated Al2O3. Oxyaluminum fluoride species on fluorinated Al surfaces are most probably due to the reactions with the residual water and hydroxyl species in the reaction chamber (kept at the base pressure of 10 4 Torr). XPS data show the formation of AlF3 species only on fluorinated aluminum, indicated by a peak at 77.5 eV. This observation is in agreement with the infrared data that show a broad envelope for fluorinated aluminum, with AlF3-related contributions at lower wavenumbers in the 530 and 870 cm 1 range. The XPS measurements are consistent with those of Zhang et al.,2 who assigned peaks at 72 and 77 eV to metallic Al and Al F for XeF2 etched Al, and peaks at 75 and 77 eV to Al in Al2O3 and Al F for XeF2 etched Al2O3, respectively. Interaction with Trace H2O at the XeF2-Etched Al Surface. To gauge the macroscopic wettability of the XeF2 processed Al surfaces, contact-angle measurements were made. XeF2 exposure results in remarkably hydrophilic substrates. Highly hydrophilic surfaces typically show water contact angles in the range from 0° to 30°, with lower angles indicating a higher propensity for H2O adsorption. The contact-angle value measured ex situ for these surfaces is ∼5° 10°, indicating a very high affinity for water. In situ IR spectroscopy reveals the interaction of trace H2O in the reactor with the surface, before exposure to atmosphere. The base reactor pressure (10 4 Torr) is not good enough to prevent residual OH and H2O to interact with the surfaces after XeF2 etching. To probe the interaction of trace H2O with XeF2 fluorinated Al and Al2O3 substrates, a D2O/H2O exchange test was performed. The samples after etching were then exposed to D2O vapor at 7.5 Torr pressure for 10 min. Figure 4 shows the differential infrared spectra obtained from the etched Al surface after exposure to D2O vapor. The spectrum clearly shows that absorbed H2O was present and associated with the AlF3/Al

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Figure 5. IR spectra obtained from XeF2-fluorinated Al surface before (dashed line) and after (continuous line) exposure to water. Spectrum (a) was referenced to the one obtained before XeF2 fluorination; spectrum (b) was referenced to spectrum (a). The water-related absorption bands are shown at 2 magnification.

surface after treatment in the reactor. The D2O/H2O exchange is characterized by the negative broad absorption band above 3000 cm 1 and appearance of the positive ν(D2O) absorption band above 2100 cm 1. This confirms the presence of H2O related species on the surfaces after the etching process was terminated. The absorption band due to the bending δ(D2O) mode shows two contributions, at 1260 and 1215 cm 1. Figure 5 shows the effects occurring at the fluorinated Al surface upon further exposure to vapor H2O. In the aluminum fluoride spectral range, exposure to water leads to the appearance of negative features at 652 and 815 cm 1 and of a positive feature at 732 cm 1. These bands are very broad, and multiple possibilities for assignments exist. One possibility is incorporation of oxygen into the Al F network due to interaction with water, in addition to surface adsorption. In this case, a shift toward higher wavenumbers can be due to the formation of AlOxFy species. The presence of these species could also be consistent with the XPS data presented in Figure 3c.

’ DISCUSSION AND COMPUTATIONAL MODELING Infrared and XPS spectroscopy provide clear evidence that the etching of a Si overlayer on Al results in the formation of an AlF3 layer upon exposure of the underlying substrate to XeF2. Spectra a2 and b2 in Figure 1 show that the fluorination of the Al substrate occurs simultaneously with the removal of the final layer of Si, and spectra a3 and a4 together with spectra b3 and b4 indicate that an AlF3 passivation layer forms rapidly and reaches a steady state on the Al/AlF3 surface. The infrared spectrum of the XeF2 etched Al surface, shows a broad absorption envelope from 530 to 870 cm 1, where there is some structure with discernible peaks at 798 and 735 cm 1. The differential infrared spectrum for fluorinated Al2O3 shows the appearance of a band at 768 cm 1, which could be associated with an AlOxFy species. XPS characterization of the Al/AlF3 and F/Al2O3 surfaces clearly shows that the AlOxFy species, characterized by a peak at 76.4 eV, evident in the F/Al2O3 IR, is common to both surfaces. The IR and XPS analyses of the XeF2 etch-prepared Al/AlF3 substrate show evidence for water adsorption at the surface. A D2O/H2O exchange experiment confirms the presence H2O-related species at the surface. The Al/AlF3 surface adsorbed D2O shows two peaks in the δ(D2O) region at 1260 and 1215 cm 1. Direct exposure of the etched Al/AlF3 surface to H2O leads to a loss of IR bands at 815 and 652 cm 1 and the appearance of a band at 732 cm 1. 21354

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Figure 6. Al/AlF3 and H2O derived surface structures calculated using GGA-DFT. Structures correspond to the computed vibrational frequencies reported in Table 2. (Al is gray, fluorine is green, oxygen is red, and hydrogen is white.)

Table 2. GGA-DFT Calculated Vibrational Frequencies and General Mode Assignments for Al/AlF3 and H2O Derived Surface Species for Comparison with Observed IR Bandsa surface structure

surface species

GGA-DFT frequencies (cm 1)

general mode assignment

a

Al/AlF3

843, 653

Al F str (t), Al F str (b)

b

Al/AlF3 3 3 3 OH2

1613, 800, 773, 733, 642

H2O scissor, H2O bend + Al F str (terminal), H2O bend + Al F str (bridging),

c d

Al/AlF2 OH2 Al/AlF2 OH

1607, 944, 788, 707, 650 848, 687, 655

H2O scissor, H2O bend, Al F str (terminal), H2O wag, Al F str (bridging) Al O str, HO bend, Al F str

e

Al/F3Al O AlF3

987, 768, 676, 619

Al O str (b), Al F str (t), Al F str (b), Al F str (b) O F str, Al O str, Al F str

H2O rock, H2O deform + Al F str (bridging)

a

Atomistic structures for surface structures a e are shown in Figure 6.

AlF3 substrates from numerous synthetic preparations have been studied due to their catalytic properties for industrially relevant processes. AlF3 is a strong Lewis acid because of the strong electron-withdrawing nature of fluorine acting on the Al metal center. Different AlF3 preparations lead to different structural phases of AlF3 that exhibit variable acidity depending on the exposure of the Al Lewis acid sites at the surface, with noncrystalline high-surface area AlF3 samples having remarkably high Lewis acidity.4,13,14 Since H2O is a Lewis base, the high Lewis acidity of AlF3 leads to very hydrophilic surfaces stemming from strong AlF3 3 3 3 H2O interactions at the atomistic scale. The XeF2 etched Al/AlF3 surfaces reported here, expected to have a high degree of disorder, show high hydrophilicity with a contact angle e 10° with evidence for Al bonding to F and O in the XPS and interactions with H2O in the measured IR. To gain guidance into the origin of the observed infrared bands associated with the Al/AlF3 and H2O derived surface species, various Al/AlF3 surface structures were modeled using DFT. Structural optimizations and subsequent Γ-phonon calculations were carried out for surface species based on a model of the XeF2-prepared Al/AlF3 substrate. The Al/AlF3 surface structure atomistic models are shown in Figure 6.29 The following

cases are considered: (a) the reference multilayer AlF3 on Al surface structure, Al/AlF3 (Figure 6a), (b) H2O adsorbed onto a stoichiometric AlF3 surface site, Al/AlF3 3 3 3 OH2 (Figure 6b), (c) H2O terminated AlF2 surface site, Al/AlF2 OH2 (Figure 6c), (d) hydroxyl ( OH) terminated AlF2 surface site, Al/AlF2 OH (Figure 6d), and (e) O bridging AlF2 surface units, Al/F2Al O AlF2 (Figure 6e). The GGA-DFT computed vibrational frequencies and mode assignments for vibrations in and around the characteristic aluminum fluoride and oxyaluminum fluoride region observed experimentally between 530 and 870 cm 1 are presented in Table 2. The simulation results provide reliable interpretations of the observed infrared spectra for the Al/AlF3 surface following XeF2 etching and upon association with H2O. As indicated above, the IR spectra and observed changes upon H2O dosing provide a key to unraveling the Al/AlF3 surface structure. The absorption envelope for the XeF2 etched Al/AlF3 surface gives two broad bands with peaks at 798 and 735 cm 1. These bands can be assigned to modes of H2O adsorbed on an undercoordinated surface AlF3 (structure b in Figure 6 and Table 2), computed at 800 and 733 cm 1. Upon additional H2O dosing, the Al/AlF3 surface spectrum shows a loss of modes observed at 21355

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The Journal of Physical Chemistry C 652 and 815 cm 1. These modes are clearly associated with the AlF3 surface layers, being directly assigned to terminal (t) and bridging (b) Al F stretching vibrations calculated at 653 and 843 cm 1, respectively (structure a). With high water exposure, Al F on the surface can undergo modification by interaction with H2O leading to a shift or loss of intensity of these modes. A mode with increased intensity observed at 732 cm 1 is well reproduced in the simulations, computed at 733 cm 1, and is assigned to the rocking vibration of the first layer of adsorbed H2O. Interestingly, the XPS spectra show that both the XeF2 exposed Al and Al2O3 substrates have a specific AlOxFy species matching the characteristic IR frequency of 768 cm 1, clearly observed in the F/Al2O3 spectrum. This mode can be uniquely assigned to a terminal fluorine stretching vibration associated with a surface Al with a bridging O to neighboring AlFx units (structure e in Figure 6 and Table 2). The calculated vibrational frequency is 768 cm 1, which is excellent agreement with the observed value. DFT calculations including additional H2O molecules hydrogen bonded to a surface adsorbed H2O indicate that the D2O peaks assigned as δ(D2O) at 1260 and 1215 cm 1 both arise from an adlayer D2O species and cannot be specifically assigned to a unique structure. AlF3 surfaces have extremely high affinities for adsorption of H2O. Due to the high electronegativity of fluorine, there is the formation of strongly polarized Al F linkages creating a strong Lewis acid, able to bind water strongly. The effect of fluorination is illustrated by calculating the H2O adsorption energy across the series of molecules: Al(CH3)3, AlF(CH3)2, AlF2(CH3), and AlF3. The computed H2O binding energies for these simple structures are 0.62, 0.87, 0.99, and 1.27 eV, respectively. Fluorination leads to an increase in the hydration energy by approximately a factor of 2. The adsorption of H2O on the Al/AlF3 surface modeled here, represented by structure b in Figure 6, is calculated to be energetically favorable by 0.81 eV. Further density functional calculations are carried out to assess the stability of possible oxyfluoride termination of AlFx surface units. Initial simulations are intriguing because the energy minimized Al O F surface structure is predicted to have a surface phonon mode at 767 cm 1, which could be taken for excellent agreement with the experimentally observed characteristic mode at 768 cm 1. However, energetic considerations rule out this possibility because the aluminum oxyfluoride structure is thermodynamically very unfavorable. The Al O F structure is calculated to be higher in energy than the O Al F structure by more than 6 eV. Ab initio molecular dynamics simulations at an elevated temperature (800 K) reflect this large difference in relative stability. Starting from an Al O F surface structure, the oxygen is observed to migrate during the simulation within ca. 650 fs into a bridging configuration forming a F3Al O AlF3 structure (structure e in Table 2 and Figure 6). This finding clearly shows that Al O F bonding is unfavorable and is not the reason for surface hydrophilicity. The work presented here shows that the etching of a sacrificial Si layer on an Al substrate leads to the formation of highly hydrophilic AlF3 layers on the Al surface. It further shows that fluorination of the Al substrate during the XeF2 etch passivates the surface, in particular against Al oxidation. This passivating effect is demonstrated by (1) the fact that once the sacrificial layer is removed and the Al surface is fluorinated, additional exposures to XeF2 produce no further changes on the surface; and (2) as previously shown,3 there is no significant change in the amount of surface AlF3 species or the Al oxidation after prolonged storage

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in air (monitored during 2 months). The very minor changes that occur in AlF3 and AlOxFy upon prolonged storage in air3 can be attributed to rough AlFx surfaces with defects that allow penetration of oxygen beneath the AlFx layer, and undercoordinated Al centers that lead to association of H2O with the surface by formation of strongly adsorbed Al-bonded and weaker bound hydrogen-bonded H2O species, as demonstrated in this paper. This work confirms and extends the understanding of the structure of AlF3 surfaces and their interactions with H2O. Using theoretical calculations as a guide, details of the bonding configuration of H2O at AlF3 surfaces has been uncovered, confirming previous theoretical and experimental studies14,30 that suggested that hydration of catalytic AlF3 surfaces is unavoidable under practical processing conditions. The high hydrophilic nature of surfaces resulting from XeF2 exposure of an Al layer must be accounted for in MEMs applications requiring low stiction and meniscus forces, and new functionalization methods must be developed.3

’ CONCLUSIONS We have shown that XeF2 etching of sacrificial silicon films from an oxide-free aluminum substrate produces AlF3 surface layers, characterized by high wettability (contact angle of ∼5 10°). The strongly hydrophilic surfaces reflect the high Lewis acidity of Al F species, resulting in strong adsorption of H2O at undercoordinated Al sites. A combination of infrared (IR), XPS, and first-principles simulations provides details into the atomistic structure, chemical states, and vibrational signatures of the hydrated AlF3 surface layers. Specifically, the fluorinated aluminum surface layer provides passivation against further reactions, in particular against aluminum oxidation. Previous studies of surface stability in air and the present studies of H2O reaction with AlF3 surfaces indicate that there are mechanisms for some limited oxygen penetration, involving defects or by formation of F3Al O AlF3 bonds. First principles calculations indicate that, for AlF3, insertion of oxygen into Al F bonds is an untenable surface termination. The formation of a hydrophilic AlF3 surface after vapor etching of Al poses a serious challenge for the release of low-stiction MEMs devices incorporating Al top layers. ’ ASSOCIATED CONTENT

bS

Supporting Information. Additional computational details. This material is available free of charge via the Internet at http://pubs.acs.org.

’ ACKNOWLEDGMENT This work was fully supported by the National Science Foundation (Grant CHE-0911197). The authors are grateful to X-M. Yan (then at Qualcomm MEMS Tech, Inc.) for providing the Al/Si samples and for stimulating discussions. ’ REFERENCES (1) Ibbotson, D. E.; Flamm, D. L.; Mucha, J. A.; Donnelly, V. M. Appl. Phys. Lett. 1984, 44, 1129. (2) Zhang, T. F.; Park, J. Y.; Huang, W. Y.; Somorjai, G. A. Appl. Phys. Lett. 2008, 93, 141905. (3) Roodenko, K.; Seitz, O.; Gogte, Y.; Veyan, J. F.; Yan, X. M.; Chabal, Y. J. J. Phys. Chem. C 2010, 114, 22566. (4) Kemnitz, E.; Menz, D. H. Prog. Solid State Chem. 1998, 26, 97. (5) Manzer, L. E. Science 1990, 249, 31. 21356

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(6) Manzer, L. E.; Rao, V. N. M. Catalytic synthesis of chlorofluorocarbon alternatives. In Advances in Catalysis; Eley, D. D., Pines, H., Weisz, P. B., Eds.; Academic Press Inc.: San Diego, 1993; Vol. 39, p 329. (7) Kleist, W.; Haessner, C.; Storcheva, O.; Kohler, K. Inorg. Chim. Acta 2006, 359, 4851. (8) Hess, A.; Kemnitz, E.; Lippitz, A.; Unger, W. E. S.; Menz, D. H. J. Catal. 1994, 148, 270. (9) B€ose, O.; Unger, W. E. S.; Kemnitz, E.; Schroeder, S. L. M. Phys. Chem. Chem. Phys. 2002, 4, 2824. (10) Boese, O.; Kemnitz, E.; Lippitz, A.; Unger, W. E. S. Surf. Sci. Spectra 1998, 5, 75. (11) Delattre, J. L.; Chupas, P. J.; Grey, C. P.; Stacy, A. M. J. Am. Chem. Soc. 2001, 123, 5364. (12) Wander, A.; Bailey, C. L.; Mukhopadhyay, S.; Searle, B. G.; Harrison, N. M. J. Mater. Chem. 2006, 16, 1906. (13) Mukhopadhyay, S.; Bailey, C. L.; Wander, A.; Searle, B. G.; Muryn, C. A.; Schroeder, S. L. M.; Lindsay, R.; Weiher, N.; Harrison, N. M. Surf. Sci. 2007, 601, 4433. (14) Bailey, C. L.; Mukhopadhyay, S.; Wander, A.; Searle, B. G.; Harrison, N. M. J. Phys. Chem. C 2009, 113, 4976. (15) Veyan, J. F.; Aureau, D.; Gogte, Y.; Campbell, P.; Yan, X. M.; Chabal, Y. J. J. Appl. Phys. 2010, 108, 7. (16) Veyan, J. F.; Halls, M. D.; Rangan, S.; Aureau, D.; Yan, X. M.; Chabal, Y. J. J. Appl. Phys. 2010, 108, 11. (17) Hafner, J. J. Comput. Chem. 2008, 29, 2044. (18) MedeA 2.5; Materials Design, Inc.: Santa Fe, NM, 2010. (19) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865. (20) Nabiev, S. Russ. Chem. Bull. 1998, 47, 535. (21) Hills, M. M. Appl. Surf. Sci. 1994, 78, 165. (22) Gross, U.; Rudiger, S.; Kemnitz, E.; Brzezinka, K. W.; Mukhopadhyay, S.; Bailey, C.; Wander, A.; Harrison, N. J. Phys. Chem. A 2007, 111, 5813. (23) Schn€ockel, H. G. J. Mol. Struct. 1978, 50, 267. (24) Snelson, A. J. Phys. Chem. 1967, 71, 3202. (25) Krahl, T.; Vimont, A.; Eltanany, G.; Daturi, M.; Kemnitz, E. J. Phys. Chem. C 2007, 111, 18317. (26) Picard, G. S.; Bouyer, F. C.; Leroy, M.; Bertaud, Y.; Bouvet, S. J. Mol. Struct.: THEOCHEM 1996, 368, 67. (27) Bulgakov, O. V.; Uvarov, A. V.; Antipina, T. V. Rus. J. Phys. Chem. 1969, 43, 475. (28) Krahl, T.; Stosser, R.; Kemnitz, E.; Scholz, G.; Feist, M.; Silly, G.; Buzare, J. Y. Inorg. Chem. 2003, 42, 6474. (29) The Al/AlFx surface structure atomistic models b e (shown in Figure 6 and referenced in Table 2) were constructed based on the optimized Al/AlF3 structure. For the case of H2O adsorption to a stoichiometric AlF3 site (structure b), the initial structure was generated by placing a H2O molecule in close proximity to the surface AlF3 unit. For the cases of H2O adsorption and OH termination on a AlF2 surface vacancy site (structures c and d), the initial structures were generated by changing one of the surface fluorines in the AlF3 structure to oxygen and then adding the required number of hydrogens. The initial bridging F3Al O AlF3 structure (structure e) was the result of a molecular dynamics simulation described later in the paper. The initial Al/AlFx structures were fully optimized until the maximum forces on the ions in the system were below 0.02 eV/Å. Subsequent Γ-point phonon calculations were carried out for the optimized structures yielding the vibrational frequencies and phonon displacement vectors for comparison to the observed IR bands. (30) Makarowicz, A.; Bailey, C. L.; Weiher, N.; Kemnitz, E.; Schroeder, S. L. M.; Mukhopadhyay, S.; Wander, A.; Searle, B. G.; Harrison, N. M. Phys. Chem. Chem. Phys. 2009, 11, 5664.

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dx.doi.org/10.1021/jp207839w |J. Phys. Chem. C 2011, 115, 21351–21357