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A-1040 Wien, Austria. ReceiVed: January 4, 2000. Ultrathin SiO2 films with thicknesses between 0.3 and 8 nm were grown on native silicon (Si/SiO2), mu...
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J. Phys. Chem. B 2000, 104, 5309-5317

5309

Monolayer-Controlled Deposition of Silicon Oxide Films on Gold, Silicon, and Mica Substrates by Room-Temperature Adsorption and Oxidation of Alkylsiloxane Monolayers T. Vallant, H. Brunner, J. Kattner, U. Mayer, and H. Hoffmann* Institute of Inorganic Chemistry, Vienna UniVersity of Technology, Getreidemarkt 9, A-1060 Wien, Austria

T. Leitner and G. Friedbacher Institute of Analytical Chemistry, Vienna UniVersity of Technology, Getreidemarkt 9, A-1060 Wien, Austria

G. Schu1 gerl, R. Svagera, and M. Ebel Institute of Applied and Technical Physics, Vienna UniVersity of Technology, Wiedner Hauptstrasse 8-10, A-1040 Wien, Austria ReceiVed: January 4, 2000

Ultrathin SiO2 films with thicknesses between 0.3 and 8 nm were grown on native silicon (Si/SiO2), muscovite mica and polycrystalline gold substrates via repeated application of a binary reaction sequence, which involved the formation of a self-assembled alkylsiloxane monolayer (step A) and UV-ozone oxidation of the hydrocarbon groups (step B). Using octadecyltrichlorosilane as a precursor, SiO2 films could be grown in a strictly linear, layer-by-layer mode on each of the three substrates with a growth rate of 3.0 ( 0.3 Å per deposition cycle, which corresponds to a monolayer of SiO2. The properties of these oxide films (composition, structure, packing density) were found to be essentially identical and independent of the substrate, as evidenced by ellipsometry, infrared reflection, and X-ray photoelectron spectroscopy. Furthermore, the quality of the oxide layers was investigated as a function of the hydrocarbon chain length of the alkylsiloxane monolayer formed in step A, using four different alkyltrichlorosilanes, R-SiCl3 (R ) C18H37, C11H23, C4H9, CH3), as precursors. For each compound, a linear increase of the SiO2 film thickness with the number of applied deposition cycles was again observed, but the growth rate increased noticeably from 2.8 Å/cycle for the C18 and the C11 compound to 3.2 Å/cycle and 6.5 Å/cycle for the C4 and the C1 compound, respectively, concomitant with an increase of the surface roughness in atomic force microscopy images of the oxide films. The packing density of the Si atoms in these films remains essentially constant for longer-chain precursors (R g C4), although the structure of the hydrocarbon layer changes drastically from a highly-ordered, perpendicular alignment on the surface (R ) C18) to a random, isotropic arrangement (R e C11). For films prepared from shorter precursors (R < C4), however, multilayer formation sets in and results in film growth rates clearly beyond the monolayer level. A minimum chain length of about four C atoms is therefore required to restrict the alkylsiloxane film formation (step A) to the monolayer level and to provide reproducible and precise control of the oxide film thickness in this deposition process.

Introduction The growth of ultrathin films with thickness control at the molecular level is an important goal in nanotechnology and, in particular, in microelectronics, where the device dimensions continue to shrink toward the molecular scale.1 For example, SiO2 gate oxide films with a thickness of less than one nanometerscorresponding to only four molecular layersswill be needed for a future generation of ultra-large-scale-integrated (ULSI) field effect transistors,2 whose industrial fabrication requires new and improved growth methods capable of producing homogeneous, pinhole-free films with atomically flat interfaces. We have recently reported on a novel procedure for a controlled monolayer deposition of silicon oxide films on native silicon (Si/SiO2) substrates.3 This method is based on a binary A-B reaction sequence shown in Scheme 1, which * Corresponding author. Fax: +43 1 58801 153 99. E-mail: hhoffman@ email.tuwien.ac.at.

involves the formation of an octadecylsiloxane monolayer by molecular self-assembling from solution (step A), followed by UV/ozone oxidation of the hydrocarbon groups (step B). This reaction cycle leaves a hydrocarbon-free film with a thickness ∆d ) 2.7 Å on the substrate, which corresponds to one monolayer of SiO2. Repeated application of this A-B reaction cycle resulted in a strictly linear growth of the oxide film with an increment of 2.7 Å per cycle.3 Compared to other growth methods for silicon oxide films (thermal oxidation,4 chemical vapor deposition (CVD),5 sequential atomic layer growth based on alternate adsorption of SiCl4 and H2O monolayers from the gas phase,6 photochemical decomposition of organosiloxane films7), the process described in Scheme 1 has several advantages: First, the entire growth takes place at room temperature, whereby unwanted diffusion and redistribution of dopants in the substrate are largely avoided. Second, densely packed alkylsiloxane monolayers can be adsorbed from suitable precursor solutions onto substrates of arbitrary size and shape, in

10.1021/jp000006a CCC: $19.00 © 2000 American Chemical Society Published on Web 05/13/2000

5310 J. Phys. Chem. B, Vol. 104, No. 22, 2000 SCHEME 1: Binary Reaction Cycle for a Layer-by-Layer Deposition of Silicon Oxide Films through Repeated Adsorption (A) and Oxidation (B) of Alkylsiloxane Monolayers

Vallant et al. confined to the hydrocarbon chains (trans-gauche isomerization) or whether the siloxane network is also affected as a result of a reduced packing density and/or differences in the intermolecular bonding (incomplete cross-linking, onset of 3D polymerization under multilayer formation) upon chain length reduction. In the latter case, a degradation in the quality of the silicon oxide layer (inhomogeneous thickness, pinholes, nonstoichiometric composition) after removal of the hydrocarbon groups must be anticipated and a layer-by-layer growth mode as shown in Scheme 1 would be corrupted. Experimental Section

particular on structured surfaces with high aspect ratios8 or on inner surfaces of porous substrate materials.9 Third, both half reactions in our A-B sequence are complete and self-limiting at the monolayer level over a wide range of experimental conditions10 (precursor concentration, type of solvent, residual water concentration, reaction time, etc.), which guarantees reproducible film growth rates and film compositions without a tedious fine-tuning of all process parameters to strictly specified values. In the present study, we extend our previous investigations with a more detailed characterization of the oxide films, including XPS and AFM measurements, and address two important questions with respect to practical applications: First, is this growth process applicable to other types of substrates besides native silicon surfacessin particular, to HO-deficient or entirely nonfunctional surfaces? Several recent studies have shown that densely packed, ordered alkylsiloxane monolayers can be formed not only on HO-rich oxide surfaces like SiO2, Al2O3, TiO2, or glass,11 but also on surfaces like gold12 or muscovite mica,13 which are known to possess few, if any, surface hydroxyl groups. This was ascribed to the presence of an ultrathin water layer and to the ability of the silanol precursor molecules to cross-polymerize,12b yielding a laterally bonded, 2D polymer that is anchored to the substrate by adventitiously present hydroxyl groups. Since the structure and packing density of these films do not seem to depend on the particular substrate, silicon oxide films of similar quality should be obtainable on a range of different substrate materials by this sequential deposition method. The second question is related to the role of the hydrocarbon chain length of the film molecules in this growth process. Since the alkyl groups are oxidized and removed in step B of this procedure, it would be desirable to use precursor molecules with the shortest possible hydrocarbon chains that still provide a reproducible layer-by-layer growth mode as did the long-chain C18 model compound used in our first report. This problem is intimately related to the structure and packing density of these monolayers as a function of the hydrocarbon chain length, which has been the subject of several previous studies.10,14 There is general agreement today that monolayer films with short hydrocarbon chains are considerably more disordered than their long-chain analogues, whereby the transition zone between the long-chain, crystalline-like structure and the short-chain, liquid-like phase lies somewhere between a C8 and a C12 alkyl group. The fundamental question for the present work was whether such structural transitions are essentially

Adsorbate Compounds and Substrates. The precursor compounds octadecyltrichlorosilane (Aldrich, 95%), butyltrichlorosilane (Aldrich, 99%), and methyltrichlorosilane (Aldrich, 99%) were obtained commercially and were used as received. Undecyltrichlorosilane was available from previous studies.10 Gold-coated glass slides, which were prepared by sputter deposition of about 200 nm of gold onto glass slides coated with a thin chromium adhesive layer, were obtained from Pharmacia (Uppsala, Sweden) and were cleaned by sonication in H2SO4/H2O2 solution (4:1 v/v), followed by rinsing with doubly distilled water, acetone, and ethanol and blow-drying in a stream of high-purity nitrogen (99.99%). Muscovite mica sheets were obtained from SPI Supplies (West Chester, PA) and were freshly cleaved with an adhesive tape immediately before use. Silicon wafers ((100) type, test grade, p-doped, 1430 Ω cm resistivity, 0.5-mm thickness) were obtained from Wacker Chemitronic and were cut into rectangular pieces of appropriate size with a diamond-tip stylus. They were cleaned by ultrasonic treatment in toluene, rinsing with acetone and ethanol, and blow-drying in nitrogen and were finally exposed to a UV/ozone atmosphere in a commercial cleaning chamber (Boekel Industries, Model UVClean) equipped with a lowpressure mercury quartz lamp. This treatment yields a hydrophilic surface with a native oxide layer of 12-14 Å thickness, as routinely checked with ellipsometry. Pure silicon substrates, which were needed as references for the IR measurements, were prepared by removal of the native oxide layer through immersion of the Si/SiO2 wafers in dilute HF solution (5% w/w) for 5 min followed by ultrasonic treatment in doubly distilled water, rinsing with ethanol, and blow-drying with nitrogen. Deposition of Silicon Oxide Films. Silicon oxide films were grown on gold, mica, and silicon substrates in a step-wise mode through repeated application of the binary reaction cycle shown in Scheme 1. The substrates were immersed into 1 mmol/L solutions of an alkyltrichlorosilane R-SiCl3 (R ) C18H37, C11H23, C4H9, and CH3) in toluene (residual water content 5-10 mmol/ L) for about 30 min and were rinsed afterwards with toluene, acetone, and ethanol, followed by blow-drying with nitrogen. On silicon substrates, the film surface was, in addition, gently wiped with a toluene-soaked tissue. On gold and mica substrates, this mechanical cleaning step led to visible scratches of the gold surface and partial removal of the monolayer on both gold and mica, so that these latter substrates were cleaned just by solvent rinses. The film-covered substrates were subsequently placed in a UV/ozone chamber (Boekel, UVClean) for 15 min, whereby the hydrocarbon groups of the alkylsiloxane films were oxidized and the cross-linked siloxane layer, terminated by HO-groups, remained on the surface. This adsorption/oxidation cycle was then repeated for a specified number of times. Ellipsometry. Film thickness measurements of alkylsiloxane and silicon oxide layers on gold and silicon substrates were carried out with a Plasmos SD 2300 ellipsometer equipped with

Deposition of Silicon Oxide Films a rotating analyzer and a He-Ne laser (λ ) 632.8 nm) at 68° incidence. The raw ellipsometric data (relative phase shift ∆ and amplitude ratio Ψ) were converted into film thicknesses using the commercial instrument software based on the McCrackin algorithm.15 A model of 3 or 4 isotropic phases (substrate/SiO2/air or substrate/SiO2/alkylsiloxane/air) was used to calculate the thicknesses of the SiO2 and alkylsiloxane films, respectively, as described in detail previously.16 Each sample was measured at 3-5 different spots, whereby the corresponding thickness variations across the sample surface never exceeded the experimental accuracy of about 1 Å for these measurements. Infrared Spectroscopy. External reflection infrared spectra with a fixed incidence angle of 80° were measured using a custom-made external reflection optical system connected to a Mattson RS1 FT-IR spectrometer, as described in detail elsewhere.17 Variable angle reflection spectra were measured with a commercial reflection unit (Seagull, Harrick Sci.) mounted inside the sample chamber of the spectrometer. A wiregrid polarizer was used to obtain polarized radiation with the electric field vector oriented either parallel (p-polarized) or perpendicular (s-polarized) to the plane of incidence at the substrate surface. For each spectrum, 1024 scans at 4 cm-1 resolution were co-added both from the sample and the reference. Calculations of surface electric fields and of model IR spectra of thin silicon oxide films were carried out with a computer program based on a semiempirical matrix formalism, which is described in detail in previous publications.10c,14a Atomic Force Microscopy. AFM measurements were accomplished with a Nanoscope III Multimode SPM (Digital Instruments, Santa Barbara, CA). The instrument was operated in tapping mode using silicon cantilevers with integrated silicontips (spring constant ∼28-52 N/m, resonance frequencies 200400 kHz). The images were recorded with a resolution of 512 × 512 pixel at a scan rate of 3 Hz over an area of 500 × 500 nm2. The surface roughness was calculated as the root-meansquare (rms) variation of the height profiles using the Nanoscope III software. X-Ray Photoelectron Spectroscopy. Silicon and mica substrates covered with SiO2 films were analyzed with a Kratos XSAM 800 spectrometer using a twin anode (Mg, Al) X-ray tube without a monochromator as excitation source. The acquired photoelectron spectra were excited by Mg KR1/2 radiation and by the satellite lines Mg KR3/4. The angle between the X-rays and the detected electrons was fixed to 70°. The mass coverage (mass per unit area) of the deposited films was calculated by a quantitative algorithm18 based on fundamental parameters taken from the literature. This algorithm is based on a layer model of the sample and consists of a calculator for the electron count rates and an iterator that changes different parameters (concentrations and layer thickness) until the error between calculated and measured count rates reaches a minimum. The fundamental parameters used are the electron binding energies,19 the photoelectron cross-sections,20 and the asymmetry parameters.21 The inelastic mean free path for photoelectrons (IMFP) was calculated with the Bethe equation, where the corresponding parameters were taken from ref 22. Results and Discussion SiO2 Film Growth from ODS Monolayers (1) On Native Silicon Substrates. Complementary to our initial study3 using ellipsometry and IR spectroscopy, XPS measurements were carried out to monitor the deposition of silicon oxide layers on native silicon (Si/SiO2) substrates via repeated adsorption and oxidation of octadecylsiloxane (ODS) monolayers according to

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Figure 1. (A) XPS spectra of the Si(2p) peaks of (a) native silicon (Si/SiO2), (b) native silicon after two SiO2 deposition cycles, and (c) native silicon after six deposition cycles. (B) Mass coverage of SiO2 on silicon calculated from the corrected Si(2p) count rates as a function of the number of applied deposition cycles.

Scheme 1. Figure 1A shows a typical set of Si(2p) signals from the uncorrected XPS spectra of the initial, native silicon substrate (d(SiO2) ) 14.8 Å), the same substrate after two deposition cycles (d(SiO2) ) 20.1 Å), and after six deposition cycles (d(SiO2) ) 32.3 Å). The two peaks in these spectra can be assigned to bulk silicon (99 eV) and to surface SiO2 (103 eV),23 whereby the first peak decreases and the second peak increases in intensity with increasing thickness of the SiO2 layer. A slight shift of the oxide Si(2p) peak in Figure 1A from 103.2 eV for the native oxide to 103.5 eV after six SiO2 deposition cycles to higher binding energies with increasing oxide thickness might indicate a change in the average oxidation state of silicon in the oxide film. A previous XPS study23 of silicon oxide films on Si(100) in the thickness range 0-12 Å showed a gradual change of the oxidation state from Si+1 to Si+4 with increasing thickness, whereby the 12-Å film still contained 36% of lower oxidation states (Si+1 - Si+3) and 64% of the final Si+4 state corresponding to the stoichiometric SiO2 composition. Subtraction of the Shirley background from the raw spectra in Figure 1A and deconvolution into two Gaussian peaks yielded the corrected peak count rates, from which the mass coverage of surface oxide was calculated, as described in the Experimental section. The results are shown in Figure 1B as a function of the number of applied deposition cycles. A strictly linear increase of the SiO2 mass coverage is observed, with an increment of 8.86 ( 0.45 × 10-8 g cm-2 per cycle. Using the density of crystalline quartz (2.65 g cm-3) for the surface oxide, this value corresponds to a thickness increase of 3.3 Å per cycle, in good agreement with the 2.9-Å increment obtained from the ellipsometric data. (2) On Muscovite Mica Substrates. Although the deposition process of Scheme 1 has been used in a recent publication24 to deposit a defined number of SiO2 layers onto muscovite mica substrates, there has been only indirect proof that the oxide film grows in the same layer-by-layer mode as on silicon. The main problem with mica as a silica-based mineral is that the properties of a deposited silicon oxide film are hardly different to the bulk substrate: The refractive indices are essentially the same, which rules out ellipsometric measurements, and the IR spectrum is essentially opaque in the SiO absorption region for both transmission or reflection measurements. Likewise, the chemical shifts of the Si(2p) peak in the XPS spectra of mica and silicon oxide are indistinguishable, and we have therefore attempted to use the relative peak intensities of the Al(2p) signal of the bulk substrate and the Si(2p) peak of SiO2 (substrate + film) to monitor the oxide growth process. Figure 2A shows the corresponding XPS peaks for (a) untreated, freshly cleaved mica, (b) mica after two deposition cycles, and (c) mica after six cycles of ODS adsorption and oxidation. Qualitatively, the

5312 J. Phys. Chem. B, Vol. 104, No. 22, 2000

Figure 2. (A) XPS spectra of the Si(2p) peaks of (a) freshly cleaved muscovite mica, (b) muscovite mica after two SiO2 deposition cycles, and (c) muscovite mica after six deposition cycles. (B) Mass coverage of SiO2 on mica calculated from the corrected Al(2p) and Si(2p) count rates as a function of the number of applied deposition cycles.

Figure 3. Ellipsometric thicknesses monitoring the growth of silicon oxide films on a gold substrate through repeated adsorption and oxidation of octadecylsiloxane (ODS) monolayers according to Scheme 1. The open circles represent the thicknesses measured after ODS adsorption (step A), and the solid circles are the measured thicknesses after UV/ozone oxidation (step B).

spectra show the expected decrease of the Al(2p) bulk signal and an increase of the Si(2p) peak intensity. To quantify these results, a layer model for untreated mica described in a previous XPS study25 was used, and the inelastic mean free path of the photoelectrons, which is not available for muscovite mica in

Vallant et al. the literature, was calculated for an SiO2 substrate. Figure 2B shows the mass coverage of SiO2 calculated from the corrected Al(2p) and Si(2p) peak countrates as a function of the number of deposition cycles. A strict linear growth is again observed, with a slope of 8.49 ( 0.27 × 10-8 g SiO2 cm-2 cycle-1; this value corresponds to a thickness increase of 3.2 Å per layer, based on the density of crystalline quartz, and is in close agreement to the results on native silicon substrates. (3) On Polycrystalline Gold Substrates. Polycrystalline gold substrates were subjected to sequential deposition cycles as shown in Scheme 1, using 1 mmol/L solutions of octadecyltrichlorosilane in toluene as precursor solutions, and the sequential growth of octadecylsiloxane (ODS) and SiO2 monolayers was monitored by ellipsometry and external reflection IR spectroscopy. Figure 3 shows the ellipsometric film thicknesses in the course of 10 adsorption/oxidation cycles. In close agreement to our previous study on silicon substrates,3 two series of data, one for the ODS covered surfaces (after adsorption, step A) and one for the SiO2 films (after oxidation, step B), were obtained, which both show a linear correlation as a function of the number of applied deposition cycles and yield two parallel lines in Figure 3 with an increment of 2.75 Å per cycle and an offset of 23.6 Å. These data imply that an ODS monolayer with an average thickness of 23.6 Å, in close agreement to reported literature data for ODS films on gold,12b is repeatedly formed in this process, from which, after oxidation of the hydrocarbon groups, 2.8 Å remain on the surface, corresponding to a monolayer of SiO2.3 Further insight into the composition and structure of the deposited films is obtained from external reflection infrared spectra, which are shown in Figure 4 for different stages of the deposition process characterized by the number of applied adsorption/oxidation (a/b) steps. Peak assignments and wavenumbers for these spectra are listed in Table 1. The high- and mid-frequency regions (3800-2700 cm-1 and 1700-1400 cm-1), which contain the CH and OH stretching and bending modes, display two complementary groups of absorptions, which indicate the presence of either a terminal hydrocarbon layer (spectra (1/0),(2/1),(3/2),(7/6)) or an HO-terminated oxide surface (spectra (3/3),(7/7),(10/10)). The first group comprises

Figure 4. IR reflection spectra of SiO2 and ODS/SiO2 films on gold in the course of the growth of SiO2 films according to Scheme 1. Each spectrum is labeled with a binary code (a/b), which represents the number of applied adsorption steps (a) and the number of applied oxidation steps (b) in the deposition process. Peak assignments and wavenumbers are listed in Table 1. The spectra were measured with p-polarized radiation at 80° incidence.

Deposition of Silicon Oxide Films

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TABLE 1: Peak Assignments and Wavenumbers (in cm-1) in IR Reflection Spectra of ODS/SiO2 Films on Gold (Figure 3) Prepared by Sequential Adsorption and Oxidation of ODS According to Scheme 1 number of adsorption/oxidation steps

a

vibration

1/0

2/1

3/2

3/3

7/6

7/7

10/10

ν(OH) νas(CH3) νas(CH2) νs(CH3) νs(CH2) δ(HOH) δ(HCH) ν(SiO)

n.o.a 2966 2924 2880 2853 n.o. 1468 1134

n.o. 2967 2924 2880 2853 n.o. 1468 1139 1201

n.o. 2966 2924 2880 2854 n.o. 1468 1140 1211

3745 n.o. n.o. n.o. n.o. 1653 n.o. 1140(sh) 1222

n.o. 2967 2923 2880 2852 n.o. 1468 1140(sh) 1230

3746 n.o. n.o. n.o. n.o. 1655 n.o. 1140(sh) 1235

3746 n.o. n.o. n.o. n.o. 1656 n.o. 1140(sh) 1235

n.o., not observed.

the CH stretching absorptions at 2966 cm-1 (νas(CH3)), 2924 cm-1 (νas(CH2)), 2880 cm-1 (νs(CH3)), and 2853 cm-1 (νs(CH2)) as well as the CH2 deformation mode δ (HCH) at 1468 cm-1. Both the peak positions and intensities of these bands indicate a substantial amount of structural disorder in the hydrocarbon layers: for a densely packed and well-ordered ODS film, the ν(CH2) absorptions appear at lower frequencies (2850 and 2918 cm-1) and have very small intensities due to the essentially parallel orientation of the vibrational dipole moments with respect to the metal surface.12b,16 Such highly ordered alkylsiloxane films on gold, however, require a special pretreatment of the substrate (formation of a transient AuOx layer followed by hydration with water12b), which was not attempted in this study. The second group of absorptions consists of two weak but highly characteristic bands at 3746 cm-1 arising from isolated or geminal hydroxyl groups (Si-OH or Si-(OH)2) on an SiO2 surface and at about 1655 cm-1 due to the (HOH) deformation mode of physisorbed water.26,27 In addition, a very broad absorption centered around 3400 cm-1 appears to grow in Figure 4 with the number of deposition cycles and might reflect an increasing total amount of unreacted, H-bonded hydroxyl groups in this stack of SiO2 layers. However, inevitable contributions from ice formation at the detector element as well as baseline instabilities do not allow conclusive assignments in this spectral region. The low-frequency region (1400-800 cm-1) of the IR reflection spectra shown in Figure 4 contains a strong absorption due to the Si-O stretching vibrations. The complex shape of this band arises both from optical effects, which are discussed later in this paper, and from the overlap of these absorptions of the “bulk” SiO2 layers and of the terminal C18H37SiOx layer, which appear at different frequencies: for a monolayer of ODS (spectrum (1/0) in Figure 4), this peak maximum lies at 1134 cm-1, in close agreement to a recent IR study of ODS monolayers on gold,12b whereas in a fully oxidized SiO2 film (e.g., spectrum (3/3) in Figure 4), the major absorption maximum is found at 1222 cm-1 and shifts to slightly higher wavenumbers with increasing SiO2 layer thickness (Table 1). A plot of the peak intensity of this latter absorption versus the number of oxidation steps is shown in Figure 5; it yields, in analogy to the SiO2 film thickness (Figure 3), a linear correlation. Thus, two independent parameterssthe SiO2 layer thickness and the ν(SiO) absorption intensitysevidence a linear, layer-by-layer growth of the oxide film, although the scattering of the data in Figure 3 and Figure 5 is somewhat larger in comparison to the same process carried out on silicon substrates.3 Occasional depositions beyond the monolayer level (e.g., the third ODS adsorption in Figure 3, corresponding to spectrum 3/2 in Figure 4), which were never observed on silicon

Figure 5. Peak intensity of the ν(SiO) absorption in Figure 4 as a function of the number of applied oxidation cycles.

substrates, might be a consequence of the structural imperfections of the ODS layers on gold (see above) and the milder cleaning procedure applied after film adsorption (solvent rinsing) instead of a final mechanical wiping, which could be applied only with silicon substrates (see Experimental section). SiO2 Film Growth from Alkylsiloxane Monolayers with Different Hydrocarbon Chain Lengths on Native Silicon Substrates. SiO2 films were grown on native silicon substrates by repeated adsorption/oxidation of alkylsiloxane monolayers according to Scheme 1, using precursor compounds R-SiCl3 with different hydrocarbon chain lengths (R ) C18H37, C11H23, C4H9, CH3). Figure 6 shows the ellipsometric film thicknesses in the course of the deposition processes carried out with these four different compounds. For each system, the thicknesses of the hydrocarbon-terminated films measured after R-SiCl3 adsorption (open circles) and of the oxide films measured after oxidation (solid circles) yield two parallel lines as a function of the number of applied deposition cycles. The slope k of these lines, which corresponds to the average thickness increase of the oxide film per deposition cycle, is essentially constant between ODS (k ) 2.75 Å) and UDS (k ) 2.77 Å) but increases to 3.17 Å for BS and to 6.5 Å for MS. The thickness offset ∆d between the hydrocarbon-terminated films and the oxide films in Figure 6 decreases with the hydrocarbon chain length of the film molecules from 26.5 Å for ODS to 16.3 Å for UDS, 9.0 Å for BS, and 8.4 Å for MS. These ∆d values are in excellent agreement with reported thicknesses of the corresponding alkylsiloxane monolayers,10a which have been shown to correlate linearily with the hydrocarbon chain length (number of carbon atoms) of the film molecules, except for the methyl siloxane film. In this latter case, film thicknesses of 6-8 Å have been found10a in close agreement to the value of ∆d ) 8.4 Å obtained from Figure 6, which amounts to about twice the expected thickness for a CH3SiOx monolayer and indicates the deposition

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Figure 6. Ellipsometric thicknesses monitoring the growth of silicon oxide films on native silicon (Si/SiO2) substrates through repeated adsorption and oxidation (Scheme 1) of different alkylsiloxane monolayers R-SiOx (R ) octadecyl (ODS), undecyl (UDS), butyl (BS), methyl (MS)). The open circles represent the thicknesses measured after alkylsiloxane adsorption (step A), and the solid circles are the measured thicknesses after UV/ozone oxidation (step B).

of bilayers rather than monolayers for this shortest-chain compound in this series.28 Accordingly, the thickness increase per deposition cycle (k value) also doubles in comparison to the longer-chain compounds. Figure 7 shows infrared external reflection spectra in the ν(CH) region (upper part) and in the ν(SiO) range (lower part)

Vallant et al. at different stages of the deposition process for the C18 (ODS), C14 (UDS), C4 (BS), and C1(MS) precursor compounds. In the CH stretching region, a highly reproducible ON/OFF pattern, that is, a characteristic and compound-specific absorption profile after each adsorption step and a featureless, flat baseline after each oxidation step, was observed for each precursor compound over up to 30 deposition cycles, which indicates the reproducible formation/removal of the corresponding hydrocarbon films upon adsorption/oxidation. Both the peak positions and the band intensities, however, show significant differences, depending on the chain length of the film molecules. The longest-chain compound, ODS, shows two strong, positive (upward-pointing) bands at 2851 (νs(CH2)) and 2919 cm-1 (νas(CH2)) and two negative (downward-pointing) features at 2879 and 2968 cm-1, which are assigned to the terminal methyl group vibrations νs(CH3) and νas(CH3), respectively. As shown previously,14a,29 this spectrum is highly characteristic for a well-ordered, densely packed monolayer of ODS on a dielectric silicon substrate with a close-to-perpendicular surface orientation of the hydrocarbon chains. In contrast, the shorter-chain compounds, UDS, BS, and MS, show essentially unchanged ν(CH3) absorptions but inverted (downward-pointing) ν(CH2) peaks (UDS and BS) at higher frequencies (2858 and 2927 cm-1), whose intensities decrease with decreasing chain length from UDS to BS and which are absent, of course, in the MS spectrum. This band inversion of the ν(CH2) absorptions in combination with their high-frequency shift indicates a substantial increase of the average chain tilt angle with respect to the surface normal and the approach of a disordered, randomized surface orientation of the film molecules.14a The ν(SiO) absorptions in these films, shown in the lower part of Figure 7, exhibit a complex, derivate shape due to a splitting into parallel, transverse-optical (TO) and perpendicular, longitudinal-optical (LO) components, as discussed in detail

Figure 7. IR reflection spectra in the CH-stretching region (upper part) and in the SiO-stretching region (lower part) of SiO2 and R-SiOx/SiO2 films on native silicon in the course of the growth of SiO2 films, according to Scheme 1, using different alkylsiloxane precursor films (R ) octadecyl (ODS), undecyl (UDS), butyl (BS), methyl (MS)). The same binary codes (a/b) as in Figure 4 were used to denote the number of applied adsorption and oxidation steps. The spectra were measured with p-polarized radiation at 80° incidence and were referenced against a clean, oxidefree silicon substrate.

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Figure 8. Integrated peak areas of the ν(SiO) absorption in Figure 7 for different alkylsiloxane precursors as a function of the number of applied oxidation cycles.

below. The TO mode gives rise to an upward-pointing peak at 1075 cm-1, and the LO mode shows up as a downward-pointing absorption around 1235 cm-1. Both peak maxima show minor frequency shifts of a few wavenumbers as a function of the layer thickness and also between the different precursors, but the overall shape of this band remains essentially unchanged from the native oxide layer (spectra (0/0) up to 23 deposited SiO2 layers (spectra 23/23), regardless of the particular precursor. Since the ν(SiO) peak shape and frequency are known to respond very sensitively to changes in the composition and structure of the SiO2 film,30 it may be concluded that this deposition process produces amorphous, “native-like” silicon oxide regardless of the type and structure of the alkylsiloxane precursor film. The ν(SiO) band intensities in Figure 7, therefore, lack any specific structural or orientational contributions (contrary to the ν(CH) intensities, for example) and can be used as a second, independent measuresin addition to the ellipsometric data of Figure 6sfor the thickness of the oxide film. Figure 8 shows a plot of the ν(SiO) peak areasintegrated between 950 and 1350 cm-1 over both the positive TO and the negative LO componentsas a function of the number of oxidation cycles for the different precursor films ODS, UDS, BS, and MS. For each compound, the intensity increases linearly with the number of applied deposition cycles, whereby the slopes of these lines show the same trend as the k values in Figure 6: the intensity increase per cycle is the same for SiO2 films prepared from ODS and UDS, is slightly higher for BS, and is about twice as high for the methylsiloxane precursor. Another question of interest is the surface morphology and the microscopic roughness of the oxide films deposited from different precursors. Ideally, in the case of a strict layer-bylayer deposition, the surface roughness should remain constant and equal to the initial roughness of the native oxide surface in the course of the growth process. Figure 9 shows AFM images of the surface of a native oxide in comparison to oxide films deposited by repetitive adsorption/oxidation of ODS, UDS, BS, and MS films. The root-mean-square (rms) roughness of the surface after 23 cycles is only marginally higher than the native oxide roughness in the case of ODS and UDS, whereas a significant increase in surface roughness from 1.8 Å to 3.1 Å is already observed for BS films after 19 cycles and from 1.8 Å to 2.4 Å for MS films after 6 cycles. Band Shape of the ν(SiO) IR Absorption on Gold and Silicon Substrates. The complex absorption profile of the ν(SiO) stretching absorption in the wavenumber range 9501350 cm-1 and the different shape of this band on gold and silicon surfaces are caused by its high intrinsic absorption

Figure 9. AFM images of silicon wafers covered with native oxide (A) and with silicon oxide layers deposited via adsorption/oxidation cycles from different alkylsiloxane precursors (dox, total oxide thickness (native + deposited oxide); rms, root-mean-square surface roughness).

strength and by the different surface selection rules that are operative on metal and dielectric substrates.31 With a maximum absorption coefficient kmax ) 2.8 at 1090 cm-1 determined for a reference sample of bulk, amorphous SiO232 and the concomitant strong change of the refractive index n (anomalous dispersion) in this wavenumber range (Figure 10A), a band splitting into a transverse optical (TO) and a longitudinal optical (LO)33 mode is observed for this absorption in thin film spectra of SiO2, as shown in Figure 11 for a 2.95-nm thick SiO2 film on gold and a 3.19-nm thick SiO2 film on silicon. This band splitting is caused by the large amplification of the perpendicular mean-square electric field 〈Ez2〉, which is inversely proportional to film ) (nfilm + ikfilm)2, on the high-frequency side of the absorption maximum (Figure 10B), where both n and k are small. This enhanced electric field 〈Ez2〉 results in an absorption maximum at 1235 cm-1 for the perpendicular dipole moment components of the adsorbate film (LO mode), whereas the parallel electric field 〈Ex2〉 is essentially constant throughout this frequency range (Figure 10B) and gives rise to a second absorption maximum for the parallel dipole moment components (TO mode) near the frequency of kmax of the isotropic reference spectrum. On a metal substrate (Figure 11A), only the LO mode can be observed, with a peak maximum at 1235 cm-1, because of the metal surface selection rule,34 whereas on a dielectric substrate (Figure 11B), both the LO and TO modes give rise to absorption bands at 1236 and 1061 cm-1 pointing in opposite directions.31 Further evidence for this assignment comes from angle-dependent IR reflection spectra shown in Figure 12, measured with s- and p-polarization of the incident radiation, in combination with spectral simulations (Figure 13) based on the isotropic reference data for n and k that was displayed in Figure 10. In the s-polarized spectra (Figure 12A), only the TO band is observed at 1064 cm-1 because the electric field vector of s-polarized light is oriented parallel to the surface and can therefore interact only with parallel vibrational modes. The bands are inverted over the whole range of incidence angles Θ, and their intensity decreases with increasing Θ in accordance

5316 J. Phys. Chem. B, Vol. 104, No. 22, 2000

Vallant et al.

Figure 12. External reflection infrared spectra of a 3.1-nm thick SiO2 film on silicon, measured with s-polarized (A) and p-polarized (B) radiation and different angles of incidence Θ.

Figure 10. (A) Absorption coefficient k and refractive index n of amorphous SiO2 in the ν(SiO) wavenumber region (interpolated data from ref 32). (B) Calculated mean square electric field amplitudes 〈Ex〉2 and 〈Ez〉2 as a function of frequency in a 3-nm thick film of SiO2 on silicon for p-polarized radiation at 80° incidence. A three-phase model (air/SiO2/Si) with the x-direction parallel to the surface and the z-direction normal to the surface, as shown in the insert, and an electric field amplitude of unity for the incoming radiation was assumed for the calculations. The frequency-dependent optical parameters n and k shown in the upper part (A) were used for SiO2, and constant values were used for air (k ) 0, n ) 1) and silicon (k ) 0, n ) 3.42).

Figure 13. Simulated IR reflection spectra of the ν(SiO) absorption in a 3.1-nm thick SiO2 film on silicon for s-polarized (A) and p-polarized (B) radiation and different angles of incidence Θ. The same optical parameters as in Figure 10 were used for the calculations.

arctan nsubstrate, of dielectric substrates (ΘB ) 73° for silicon) and is caused by the abrupt phase change of the electric field vector upon reflection at Θ ) ΘB.31 Summary and Conclusions

Figure 11. ν(SiO) absorption in external reflection infrared spectra of a 2.94-nm thick film of SiO2 on gold (A) and a 3.1-nm thick SiO2 film on silicon (B). The spectra were measured with p-polarized radiation at an incidence angle of 80°.

with previous results.31 This behavior is nicely reproduced in the simulated spectra (Figure 13A). Deviations in the absolute band intensities and the exact peak positions between the experimental and calculated spectra are believed to arise from differences in the optical properties (k and n values) between the thin film SiO2 samples and the literature reference data used for the calculations. The experimental p-polarized spectra (Figure 12B) show, in addition to the opposite band directions for the TO and the LO modes, an inversion of these bands between 60° and 80° incidence, which is also fully confirmed in the calculated spectra (Figure 13B). This band inversion has been shown previously to occur at the Brewster angle, ΘB )

In this study, we have investigated a growth method for ultrathin silicon oxide films that is based on the repeated application of a binary reaction cycle consisting of (A) the formation of a self-assembled alkylsiloxane monolayer R-SiOx and (B) the photochemical oxidation of the alkyl groups R. This process was studied with a long-chain precursor compound C18H37SiCl3 on three different substratessnative silicon (Si/ SiO2), muscovite mica, and polycrystalline gold. The combined result of IR reflection, XPS, and ellipsometric measurements is that the oxide films grow in the same layer-by-layer mode on each of these substrates with a strictly linear growth rate of 3 ( 0.3 Å per cycle, which corresponds to a monolayer of SiO2. Because of the largely different chemical properties of these substrate surfaces, this result strongly supports a previously postulated, substrate-decoupled film formation mechanism35 for the alkylsiloxane monolayers in the first step of this deposition process, in which the film molecules adsorb and assemble on an ultrathin water interface layer and thereby achieve very similar packing densities and geometrical structures on different substrates. The second problem addressed in this study was the dependence of this film growth process on the hydrocarbon chain length of the precursor molecules. Among four different alkyltrichlorosilanes with a C18, a C11, a C4, or a C1 alkyl group, only the C1 compound CH3SiCl3 yielded substantially thicker films of about twice a monolayer SiO2 per cycle with

Deposition of Silicon Oxide Films an increased surface roughness compared to the initial, native silicon substrate, indicating that the CH3SiOx film formation no longer saturates at the monolayer level. A slight increase in the growth rate beyond the monolayer step, concomitant with an increased surface roughness, was still observed for the C4 compound, whereas the oxide films grown from the C11 and the C18 presursors were essentially indistinguishable, both in their thicknesses and in their surface morphologies. These results are somewhat unexpected, as they do not correlate with the structure of the hydrocarbon layer formed in step A. The IR data clearly show that only the C18 compound on silicon repeatedly forms a highly ordered monolayer with close-toperpendicular aligned hydrocarbon chains on the surface, whereas the C18 film on gold is already significantly more disordered and all the shorter-chain films exhibit essentially randomly oriented alkyl groups. This order-disorder transition in the hydrocarbon layer is not paralleled, as in other self-assembled monolayer systems (e.g., organothiols36 or organophosphonates37), by a decrease in the packing density (i.e., the surface concentration of silicon atoms), which would necessarily result in a reduced growth rate of the SiO2 films. On the contrary, the growth rate remains constant between a highly ordered hydrocarbon film (ODS on silicon) and a disordered film (UDS in silicon) and starts to increase for the disordered butyl and methyl substituted alkylsiloxane films. Thus, the packing density for the longer-chain compounds appears to be determined by the quasi-2D cross-polymerization of the R-Si(OH)3 intermediates and remains constant until, for very short (C1-C4) hydrocarbon chains, 3D polymerization and multilayer formation set in. Since the 2D siloxane network of alkylsiloxane monolayers contains a significant portion of uncondensed SiOH moieties38 as potential reaction sites for 3D polymerization, the longer-chain alkyl substituents (>C4) might simply protect these SiOH groups and act as a barrier against multilayer formation. References and Notes (1) 1997 National Technology Roadmap for Semiconductors; The Semiconductor Industry Association: San Jose, CA, 1997. (2) (a) Muller, D. A.; Sorsch, T.; Moccio, S.; Baumann, F. H.; EvansLutterodt, K.; Timp, G. Nature 1999, 399, 758. (b) Tang, D.; Wallace, R. M.; Seabaugh, A.; King-Smith, D. Appl. Surf. Sci. 1998, 135, 137. (3) Brunner, H.; Vallant, T.; Mayer, U.; Hoffmann, H. Langmuir 1996, 12, 4614. (4) Boultadakis, S.; Logothetidis, S.; Papadopoulos, A.; Vouroutzis, N.; Zorba, P.; Girginoudi, D.; Thanailakis, A. J. Appl. Phys. 1995, 78, 4164. (5) Barron, A. R. In CVD of Nonmetals; Rees, W. S., Ed.; VCH: Weinheim, 1996; pp 262-282. (6) Sneh, O.; Wise, M. L.; Ott, A. W.; Okada, L. A.; George, S. M. Surf. Sci. 1995, 334, 135. (7) (a) Mirley, C. L.; Koberstein, J. T. Langmuir 1995, 11, 1049. (b) Tada, H. Langmuir 1996, 12, 4614. (8) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1998, 37, 550. (9) (a) Brandriss, S.; Margel, S. Langmuir 1993, 9, 1232. (b) Tada, H.; Nagayama, H. Langmuir 1994, 10, 1472. (10) (a) Wasserman, S. R.; Tao, Y.; Whitesides, G. M. Langmuir 1989, 5, 1074. (b) Brunner, H.; Vallant, T.; Mayer, U.; Hoffmann, H. J. Colloid

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