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Langmuir 1996, 12, 4614-4617
Stepwise Growth of Ultrathin SiOx Films on Si(100) Surfaces through Sequential Adsorption/Oxidation Cycles of Alkylsiloxane Monolayers H. Brunner, T. Vallant, U. Mayer, and H. Hoffmann* Department of Inorganic Chemistry, Technical University of Vienna, Getreidemarkt 9, A-1060 Wien, Austria Received April 23, 1996. In Final Form: July 26, 1996X A novel procedure for a controlled monolayer growth of silicon oxide films on silicon substrates is presented. It is based on a binary A-B reaction sequence involving the formation of an alkylsiloxane monolayer through self-assembling from solution (step A) followed by UV/ozone oxidation of the hydrocarbon groups (step B). Repeated application of this A-B cycle results in a layer-by-layer growth of the oxide film with a strictly linear thickness increase of 2.7 Å per cycle, as evidenced by ellipsometry and infrared reflection spectroscopy.
Introduction
Experimental Methods
Ultrathin films of silicon oxide play an important role in the semiconductor industry as insulating and dielectric layers. The fabrication of novel microelectronic quantum structure devices,1 for example, requires homogeneous, pinhole-free oxide layers of electronic tunneling dimensions (layer thickness 1-5 nm), which are difficult to produce by conventional vapor deposition methods or by thermal oxidation of silicon. Recent efforts have aimed at low-temperature deposition methods where the film thickness can be controlled at the monolayer level and unwanted diffusion and redistribution of dopants in the substrate is largely avoided.2-5 George et al.2 have used a binary reaction sequence of exposing a Si(100) surface in ultrahigh vacuum alternatingly to SiCl4 and H2O vapor at 600 K to grow SiO2 films at a rate of approximately 1.1 Å per cycle. Mirley and Koberstein3 have exposed a Langmuir-Blodgett film of poly(dimethylsiloxane) to a UV-ozone atmosphere at room temperature and obtained a 14 Å thick film of silicon oxide with 10% residual hydrocarbon content. Tada4 has used a similar treatment (UV irradiation in atmospheric oxygen) of monolayers of tetramethylcyclotetrasiloxane (TMCTS) on TiO2 to obtain a SiOx monolayer (d ∼ 2 Å). Repeated TMCTS adsorption and UV exposure gave increasingly thicker layers per cycle due to incomplete oxidation of the precursor TMCTS, which was ascribed to a photocatalytic effect of the TiO2 substrate, which rapidly decays with increasing film thickness. In this report we describe a novel room-temperature procedure based on the repeated chemisorption and oxidation of alkylsiloxane monolayers on silicon surfaces. Because of the self-limiting nature of each reaction step to one monolayer per cyclessimilar to atomic layer epitaxy (ALE) processes6sa strict layer-by-layer film growth takes place, thereby providing accurate control over the total film thickness through the number of adsorption/oxidation cycles.
Adsorbate Compounds and Solvents. Octadecyltrichlorosilane (CH3(CH2)17SiCl3, OTS) was obtained from Aldrich (95% purity) and was used without further purification. Methyl 11(trichlorosilyl)undecanoate (CH3OOC(CH2)10SiCl3, MTSUD) was synthesized by radical addition of trichlorosilane (HSiCl3, Merck, 99% purity) to methyl 10-undecenoate (CH3OOC(CH2)8CHdCH2, MUD) analogous to the preparation described previously for unsubstituted alkyltrichlorosilanes.7 MUD was obtained from 10-undecenoic acid (Aldrich, 98%) by refluxing with methanol/ sulfuric acid following a literature procedure for methyl 22tricosenoate8 and was distilled in vacuum prior to its further reaction (bp 75-80 °C/0.03 Torr, nD20 ) 1.4393). The final product, MTSUD, was isolated in 51% yield as a clear liquid through vacuum distillation (bp 114-122 °C/0.3 Torr, 1H-NMR (CDCl3) δ 3.66 (s, 3H), 2.30 (t, 2H), 1.65-1.15 (m, 18H)9). LiAlH4 was applied for the in-situ surface reduction of MTSUD as a 0.1 mol/L solution in tetrahydrofuran (Aldrich). Hydrochloric acid (Aldrich, 37%), toluene (Aldrich, 99%), acetone (Aldrich, 99%), ethanol (Austria Hefe AG, 99.9%), and n-hexane (Aldrich, 95%) were used as received. Sample Preparation. p-Doped, (100)-oriented, and singlesided polished silicon wafers (Wacker Chemitronic, test grade, 14-30 Ω cm resistivity, 0.5 mm thickness) were cut into 25 × 20 mm2 pieces and were cleaned by ultrasonic treatment in toluene (1 min), rinsing with acetone and ethanol, blow-drying in high-purity nitrogen (99.99%), and a final 15 min exposure to a UV/ozone atmosphere in a commercial cleaning chamber (Boekel Industries, Model UVClean) equipped with a lowpressure mercury quarz lamp (λmax ) 185 and 254 nm, intensity ) 0.28 mW cm-2). This treatment yields a hydrophilic, contamination-free surface with a native oxide layer of 12-14 Å thickness, as routinely checked by ellipsometry. Oxide-free silicon substrates, which were used as the reference for the IR measurements, were obtained by immersing the substrates after the above treatment 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. The cleanliness of the surface after this treatment was checked by ellipsometry (see below). Monolayers of octadecylsiloxane were formed on the oxidecovered substrates by immersing them in 1 mmol/L solutions of OTS in water-saturated toluene (water content ∼2 × 10-2 mol/ L) for about 30 min. After removal from the adsorbate solution the substrates were gently scrubbed with a toluene-soaked tissue to remove weakly-bound multilayers and underwent the standard cleaning procedure (1 min of ultrasonic treatment in toluene, rinsing with acetone and ethanol, and drying in nitrogen), after
X Abstract published in Advance ACS Abstracts, September 1, 1996.
(1) Bauer, G., Kuchar, F., Heinrich, H., Eds. Low-Dimensional Electronic Systems; Springer Series in Solid State Science; Springer: Berlin, 1992; Vol. 111. (2) Sneh, O.; Wise, M. L.; Ott, A. W.; Okada, L. A.; George, S. M. Surf. Sci. 1995, 334, 135. (3) Mirley, C. L.; Koberstein, J. T. Langmuir 1995, 11, 1049. (4) (a) Tada, H. Langmuir 1995, 11, 3281. (b) Tada, H. Langmuir 1996, 12, 966. (5) Danner, J. B.; Vohs, J. M. Langmuir 1994, 10, 3116. (6) Suntola, T. Mater. Sci. Rep. 1989, 4, 261.
S0743-7463(96)00395-2 CCC: $12.00
(7) Hoffmann, H.; Mayer, U.; Krischanitz, A. Langmuir 1995, 11, 1304. (8) Tillman, N.; Ulman, A.; Penner, T. L. Langmuir 1989, 5, 101. (9) Wasserman, S. R.; Tao, Y. T.; Whitesides, G. M. Langmuir 1989, 5, 1074.
© 1996 American Chemical Society
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Langmuir, Vol. 12, No. 20, 1996 4615
which the ellipsometric and IR measurements were carried out immediately. Multilayers of MTSUD were prepared by a repetitive procedure described previously by Sagiv et al.10 and Ulman et al.:8 The cleaned substrates were immersed in 1 mmol/L solutions of MTSUD in n-hexane for 10-100 min11 and were wiped and cleaned afterward as described above. The thicknesses of the obtained ester-terminated films were measured to ascertain complete removal of multilayer deposits. The surface ester groups were reduced to hydroxyl groups by immersing the substrates in 0.1 mol/L LiAlH4 solution in THF (15 min), rinsing with ethanol, hydrolysis in 20% HCl (15 min), rinsing with distilled water and acetone, and applying the standard cleaning procedure, after which the thickness of the obtained OH-terminated film was measured immediately. By repetition of this MTSUD adsorption and reduction cycle, a multilayer consisting of a defined number of single layers was prepared. Infrared Measurements. External reflection infrared (ERIR) spectra were measured with a custom-made reflection optical system connected to a Mattson RS FT-IR spectrometer as described in detail elsewhere.12 p-Polarized light at an incidence angle of 80° was used, and 1024 scans at 4 cm-1 resolution were averaged in each measurement from both the sample and the clean silicon reference. The interferograms were apodized using triangular apodization and were zero-filled to yield spectra with one data point per wavenumber. Ellipsometric Measurements. Ellipsometric film thickness measurements were carried out on a Plasmos SD 2300 ellipsometer with a rotating analyzer and a He-Ne laser (λ ) 632.8 nm) at 68° incidence as the light source. The ellipsometric angles (relative phase shift ∆ and amplitude ratio Ψ) were converted into film thicknesses using the commercial instrument software which is based on the McCrackin algorithm.13 Calculations of the oxide film thickness were based on an isotropic three-phase model consisting of silicon (refractive index n ) 3.865, absorption coefficient k ) 0.020), silicon oxide (n ) 1.465, k ) 0), and air (n ) 1, k ) 0). The HF-treated, clean silicon substrates, which were used as reference for the IR measurements, gave residual thicknesses of a few Å using this three-phase model. Alternatively, a two-phase model consisting of two semiinfinite media (silicon and air), which allows the determination of the substrate’s optical constants (n, k) from the measured ellipsometric angles, essentially yielded the above quoted reference values for nSi and kSi after HF treatment, from which we conclude that the residual thickness on these substrates is due to minor surface contaminants. A four-phase model (Si/SiOx/organosilane/air) was used for calculating the thickness of the organosilane layers, for which the optical constants n ) 1.50 and k ) 0 were taken from the literature.8,14 Each sample was measured at three different spots, and the corresponding thickness variations across the sample surface were always below 1 Å.
Results and Discussion SiOx Film Growth through Adsorption and Oxidation of ODS Monolayers. Scheme 1 outlines the procedure which was used to grow silicon oxide films up to ∼70 Å thick in a layer-by-layer fashion on Si(100) substrates. Starting with a Si/SiOx substrate with a native SiOx oxide layer, whose thickness lies between 12 and 14 Å (determined ellipsometrically), a monolayer of octadecylsiloxane (ODS) is formed through adsorption of OTS (10) Pomerantz, M.; Segmu¨ller, A.; Netzer, L.; Sagiv, J. Thin Solid Films 1985, 132, 153. (11) With increasing layer number, n, shorter immersion times (∼10 min) were used for the preparation, since the higher layers tended to form less reproducible films with larger thickness variations. A similar behavior was reported for multilayer films from CH3OOC(CH2)22SiCl3 (ref 10) and was ascribed to changes in the film structure and/or surface coverage with increasing film thickness. (12) Hoffmann, H.; Mayer, U.; Brunner, H.; Krischanitz, A. Vib. Spectrosc. 1995, 8, 151. (13) (a) McCrackin, F. L.; Passaglia, E.; Stromberg, R. R.; Steinberg, H. L. J. Res. Natl. Bur. Stand., Sect. A 1963, 67, 363. (b) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarized Light; NorthHolland: Amsterdam, 1977. (14) Parikh, A. N.; Allara, D. L.; Azouz, I. B.; Rondolez, F. J. Phys. Chem. 1994, 98, 7577 and references cited therein.
Scheme 1. Sequential Procedure for the Layer-by-Layer Growth of a Silicon Oxide Film on a Si(100) Surface Covered with a Native Oxide Layer (d ∼ 13 Å) through Repeated Adsorption (Step A) and UV-Ozone Oxidation (Step B) of Alkylsiloxane Monolayers (R ) C18H37)
(step A), as described in numerous previous investigations.14 By exposure of this sample to ozone, produced from atmospheric oxygen and short-wave UV radiation,15 the hydrocarbon groups are oxidized (step B) and removed from the surface as gaseous products (CO, CO2, H2O),15 while the SiOx framework of the monolayer film is left behind and causes a nominal thickness increase of one monolayer of SiOx as compared to the initial substrate. Since a hydrophilic, OH-terminated surface is produced by this treatment,16 the whole process of adsorption and oxidation can be repeated to grow a second, third, etc., layer of silicon oxide. Figure 1 shows the ellipsometric film thicknesses in the course of 23 adsorption/oxidation cycles. In each step, constant thickness increments of 26.38 ( 0.91 Å upon ODS adsorption and 2.75 ( 0.31 Å after hydrocarbon oxidation were obtained (average values over 23 cycles). The former value corresponds to the calculated length of an all-trans octadecylsiloxy chain,9,14 which indicates that densely packed ODS monolayers with the hydrocarbon chains oriented perpendicular to the surface are formed repeatedly. Likewise, the latter value equals the edge length of an [SiO4] tetrahedron and can be ascribed to the thickness of one SiOx monolayer.17 Linear fits of the data in Figure 1 for the ODS-covered surfaces and the bare oxide surfaces yield two parallel lines with correlation coefficients R2 > 0.998, thus demonstrating that even after 23 adsorption/oxidation cycles the thickness still increases strictly by one monolayer per step. The resulting total thickness of the oxide layer dSiO (Å) can be expressed as
dSiO ) dSiOnat + 2.75n
(1)
where dSiOnat is the native oxide thickness and n is the number of adsorption/oxidation cycles. Figure 2 shows the results of monitoring this film growth process by infrared reflection spectroscopy. The IR spectra in the CH stretching region show initially a hydrocarbon-free surface (spectrum a) upon which OTS is adsorbed (spec(15) Vig, J. R. In Treatise on Clean Surface Technology; Mittal, K. L., Ed.; Plenum Press: New York, 1987; Vol 1. (16) Frantz, P.; Granick, S. Langmuir 1992, 8, 1176. (17) The length of an [SiO4] tetrahedron edge is 2.65 Å based on a Si-O bond length of 1.62 Å (Wells, A. F., Structural Inorganic Chemistry, 5th ed.; Clarendon Press: Oxford, 1984). Since the structure of the SiOx residue after hydrocarbon oxidation is most likely amorphous (Ourmazd, A.; Taylor, D. W.; Rentschler, J. A. Phys. Rev. Lett. 1987, 59, 213) and the stoichiometry appears to change with distance from the Si/SiOx interface (see ref 5), the monolayer thickness of this SiOx layer is not well-defined. The edge length of an [SiO4] tetrahedron, however, represents an upper boundary for the thickness of any irregular two-dimensional array of [SiO4] tetrahedrons on the surface.
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Letters Scheme 2. Reaction Sequence for the Controlled Deposition of a Defined Number, n, of Silicon Oxide Monolayers on a Si(100) Surface Covered with a Native Oxide Layera
Figure 1. Ellipsometric thicknesses monitoring the layer-bylayer growth of a silicon oxide film on a Si(100) surface through repeated adsorption of octadecylsiloxane (ODS) monolayers followed by UV-ozone oxidation of the hydrocarbon groups according to the procedure shown in Scheme 1. Each data point represents the average value over three different spots across the sample surface, for which the variation was always below 1 Å. Linear regression of the experimental data for the oxide film (full circles) and the ODS-covered oxide film (open circles) yields two parallel lines with a slope of 2.75 Å representing the average thickness increase per adsorption-oxidation cycle.
Figure 2. ERIR spectra in the CH- and SiO-stretching region monitoring the stepwise growth of SiOx layers on Si(100) according to the procedure shown in Scheme 1. The spectra represent different stages in the oxide layer growth process characterized by the number of adsorption steps A and the number of oxidation steps B (Scheme 1 and Figure 1): (a) A ) 0, B ) 0; (b) A ) 1, B ) 0; (c) A ) 6, B ) 6; (d) A ) 16, B ) 15; (e) A ) 23, B ) 23. The insert shows the integrated intensities of the ν(SiO) absorptions between 950 and 1350 cm-1 as a function of the ellipsometric thickness of the oxide layer.
trum b), yielding the characteristic spectrum of a highly ordered ODS monolayer on silicon with close-to-perpendicularily aligned hydrocarbon chains.7 The positive (upward-pointing) bands at 2851 and 2919 cm-1 are assigned to νs(CH2) and νas(CH2), respectively, and the downward-pointing (negative) bands at 2879 and 2968 cm-1 to the terminal methyl group vibrations νs(CH3) and νas(CH3) (see ref 7 for a detailed spectral interpretation). These absorptions disappear completely upon hydrocarbon oxidation and reappear in identical manner upon adsorption of a new ODS monolayer, as shown in spectrum d after 16 adsorption steps. A flat baseline with no signs
a Step A involves the formation of an OH-terminated multilayer comprised of n [-(CH2)11SiOx] monolayers (n ) 1-5) through n times adsorption and reduction of the ester compound CH3OOC(CH2)10SiCl3 (see refs 8 and 10). In step B the hydrocarbon groups in the multilayer are oxidized by UVozone treatment, leaving behind the SiOx framework of the multilayer on the surface.
of residual hydrocarbons accumulating in the oxide layer is obtained in the ν(CH) region after 23 cycles (spectrum e), indicating essentially complete hydrocarbon oxidation throughout this experiment. The ν(SiO) region in the IR reflection spectra shows fairly complex, derivative-shaped band profiles resulting from the splitting of TO (transverse optical) and LO (longitudinal optical) modes.18 A detailed discussion of these spectral features is beyond the scope of this letter and will appear in an upcoming publication. The overall band shape of the ν(SiO) absorptions does not change in the course of the oxide layer growth apart from small band shifts of the peak maxima. The insert of Figure 2 shows that the area under the ν(SiO) absorptions grows linear with the ellipsometric thickness of the oxide layer or, equivalently, with the number of adsorption/oxidation cycles, providing additional independent evidence for a layer-by-layer growth of the oxide film. Due to sensitivity limitations, other vibrations such as ν(OH) and ν(Si-OH), which are readily seen with powdered, high-surface-area samples,4,19 could not be detected here. SiOx Films from Multilayers of ω-Hydroxyundecylsiloxane. Some preliminary experiments were carried out to investigate the oxidation of multilayer films consisting of a periodic assembly of cross-linked siloxane layers separated by hydrocarbon layers with a defined hydrocarbon chain length. These multilayers were prepared according to previous reports by Sagiv10 and Ulman.8 Scheme 2 shows the procedure of formation and oxidation of these multilayer films: A trichlorosilane compound with a terminal ester group (MTSUD) is adsorbed onto a Si/ SiOx substrate to form a siloxane monolayer, whose outer surface is comprised of ester groups. Reduction of this film surface with LiAlH4 converts the ester to the alcohol and produces surface OH-groups,8,10 which serve as anchor groups for the next ester layer. n-times repetition of this (18) (a) Berreman, D. W. Phys. Rev. 1963, 130, 2193. (b) Wong, J. S.; Yen, Y. Appl. Spectrosc. 1988, 42, 598. (c) Yen, Y.; Wong, J. S. J. Phys. Chem. 1989, 93, 7208. (d) Yamamoto, K.; Ishida, H. Appl. Spectrosc. 1994, 48, 775. (19) (a) Brandriss, S.; Margel, S. Langmuir 1993, 9, 1232. (b) Tripp, C. P.; Hair, M. L. Langmuir 1995, 11, 149. (c) Tripp, C. P.; Hair, M. L. Langmuir 1995, 11, 1215.
Letters
Figure 3. Ellipsometric thicknesses of multilayers of HO[(CH2)11SiOx]n (n ) 1-5, open circles) on Si(100) prepared by the procedure shown in Scheme 2 (step A) and of the remaining oxide layer (full circles) after UV-ozone oxidation of the hydrocarbon groups (step B). The data represent the values for two different samples measured at three different spots on each sample surface. The straight line fitted through the data for the oxide layers yields a thickness increase of 2.72 Å per layer.
adsorption/reduction cycle produces a multilayer film which consists of n[(CH2)11SiOx] layers terminated by surface OH-groups (step A). If the hydrocarbon groups were completely oxidized in the following UV-ozone treatment (step B), n SiOx layers should remain on the surface in analogy to the monolayer experiments described above. Thus, the thickness of the silicon oxide layer on
Langmuir, Vol. 12, No. 20, 1996 4617
top of the native oxide would be determined solely by the number of layers in the multilayer precursor film produced in step A. Figure 3 shows the ellipsometric film thicknesses of the multilayer films versus the number of constituent monolayers before and after oxidation in a UV-ozone atmosphere. The multilayer thickness initially increases linearily with the layer number with an increment ∆d of about 13 Å per layer for n ) 1-3, which increases to ∆d ∼ 20 Å for thicker films (n ) 4, 5) paralleled by larger thickness variations across the sample surface. A similar behavior has been described for multilayer films from CH3OOC(CH2)22SiCl38 and was ascribed to changes in the film structure and/or surface coverage with increasing film thickness. Oxidation of these multilayer films yields a strictly linear increase of the residual layer thickness dSiO ) 2.72n with the layer number n (correlation coefficient R2 ) 0.997), whereby the increment of 2.72 ( 0.32 Å per layer is, within experimental uncertainty, identical to the results obtained from ODS monolayers (see previous section) and corresponds to the thickness of a single layer of silicon oxide. At the present time we have no experimental evidence regarding the molecular details of this latter oxidation reaction, in the course of which the siloxane framework of the multilayer films, which is initially separated by hydrocarbon spacer layers, “melts” together to form a homogeneous multilayer oxide film. Corresponding studies on the mechanistic aspects of this process along with a more detailed characterization of the resulting oxide layers are currently in progress in our group. Acknowledgment. This work was supported by the Fonds zur Fo¨rderung der Wissenschaftlichen Forschung (Proj. No. P 9749) and the Hochschuljubila¨umsstiftung der Stadt Wien. LA960395T
