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Reaction Mechanisms of the Atomic Layer Deposition of Tin Oxide Thin Films Using Tributyltin Ethoxide and Ozone Charith E Nanayakkara, Guo Liu, Abraham Vega, Charles L. Dezelah, Ravindra K. Kanjolia, and Yves J. Chabal Langmuir, Just Accepted Manuscript • Publication Date (Web): 23 May 2017 Downloaded from http://pubs.acs.org on May 26, 2017
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Reaction Mechanisms of the Atomic Layer Deposition of Tin Oxide Thin Films Using Tributyltin Ethoxide and Ozone Charith E. Nanayakkara†, Guo Liu‡, Abraham Vega†, Charles L. Dezelah‡, Ravindra K. Kanjolia‡, Yves J. Chabal†* †Department of Materials Science & Engineering, The University of Texas at Dallas, Richardson, Texas 75080, United Sates ‡EMD Performance Materials, 1429 Hilldale Avenue, Haverhill, MA, 01832, United
States
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Abstract Uniform and conformal deposition of tin oxide thin films is important forseveral applicationsin electronics,gas sensing and transparent conducting electrodes. Thermal Atomic layer deposition (ALD)is often best suited for theseapplications, but its implementation requires a mechanistic understanding of the initial nucleation and subsequent ALD processes. To this end,in-situFTIR and ex-situ XPS have been used to explore the ALD of tin oxide films using tributyltin ethoxide and ozone on an OH-terminated,SiO2-passivatedSi(111) substrate.Direct chemisorption of tributyltin ethoxide on surface OH groupsand clear evidence for subsequent ligand exchangeare obtained, providing mechanistic insight. Upon ozone pulse, the butyl groups react with ozone forming surface carbonate and formate. The subsequenttributyltin ethoxide pulse removes thecarbonate and formatefeatureswith the appearance of the bands for CH stretching and bending modes of the precursor butyl ligands. This ligand-exchange behavior is repeated for subsequent cycles, as is characteristic of ALD processes, and is clearly observed fordeposition temperatures of 200 °C and 300 °C. Based on thein-situ vibrational data,a reaction mechanism for the ALD process of tributyltin ethoxide andozone is presented, whereby ligands are fully eliminated.Complementary ex-situ XPS depth profiles confirm that the bulk of the films is carbon free, i.e. formate and carbonate are not incorporated into the film during the deposition process, and that good quality SnOx films are produced. Furthermore, the process was scaled up in a cross flow reactor at 225°C, which allowed the determination of the growth rate (0.62 Å/cycle) and confirmed a self-limiting ALD growth at 225 °C and 268°C. An analysis of the temperature dependence data reveals that growth rate increases linearly between 200 °C and 300 °C.
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Introduction SnO2 is a transparent n-type semiconductor with applications in electronics, gas sensing, flat panel display and solar cells.1-4 For applications in which conformal deposition of ultrathin films is required, adequate control can only be achieved with atomic layer deposition (ALD).Many non-alkyltin ALD precursors have worked well for the low temperature deposition (≤ 250°C) of SnOx films.5-9 However, for higher temperature deposition that is preferable for obtaining high quality SnO2 films, only SnCl4has been routinely used.10,
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However, non-
halogenated ALD precursors are more desirable to avoid contamination of the films, corrosive byproduct formation, and possible particle formation. Consequently, alkyl tin precursors have gained much attention during the last decade not only due to the lack of corrosive halogen byproducts but also due to their air and water stability and high volatility.12, 13For instance, recent evidence suggests that ALD of tin oxide films using alkyltin precursors produces impurity-free films with anALD processcompatible with reasonably high-temperature deposition (300°C or higher),12, 14albeit a narrow ALD window of 290 °C and 320 °C.12 Given the increasing importance of tin oxide thin film deposition using ALD, we have explored tin oxide deposition using tributyltin ethoxide and ozone. In-situ FTIR spectroscopy was used to monitor the initial nucleation andligand exchange during the ALD process and ex-situ Xray photoelectron spectroscopy (XPS) to check the film composition and carbon contamination of the deposited films. Furthermore, thicker films were grown with tributyltin ethoxide and ozone using a cross-flow ALD reactor to characterize the growth rates and electrical resistivity, and to determine whether the surface reactions are self-limiting. Materials and Methods
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For this study,a home-built ALD reactorwas interfaced with a Fourier transform infrared (FTIR)interferometer for in-situabsorption measurements. The base pressure of the reactor was~10-4 Torr.The IRabsorption spectra wererecorded with a Thermo Nicolet 6700 spectrometer (frequency range of 400 - 4000 cm-1) equipped with a mercurycadmium telluride (MCT/B) detector and KBr beam splitter.Gate valves in the ALD reactor wereused to protect the KBr windows during precursor pulses.The Si(111) samples werecut into 3.8×1.5 cm2pieces (0.05cm thick) from a double-sided polished, float-zonegrown, and lightly doped (ρ~10 Ωcm) silicon wafer. The oxidized samples werethen subjected to a three-step degreasing sequence consisting of consecutive sonication in dichloromethane, acetone, and methanol for a duration of 5 minutes in each solvent. The thin thermal oxide on Si(111)was further cleaned in a piranha solution consisting of (1:3 H2O2:H2SO4) at 80°C for 30 minutes followed by a thorough deionized water rinsing and blow-drying with N2 gas, resulting in a clean oxide terminated with a high concentration of surface hydroxyl groups. These clean OH-terminated samples were thenintroduced into the N2-purged ALD reactor. Tributyltin ethoxide (Thermogravimetric Analysis (TGA) on Figure S1) is commercially available and was supplied by EMD Performance Materials. For the experiments performed in the ALD reactor with in situ infrared spectroscopy the following deposition conditions were used. Tributyltin ethoxide precursor was vapor drawn from an ampoule kept at 95 °C and pulsed into the reactor(1sec pulse). Ozone (O3) was produced from oxygen using an In-USA Ozone generator with controlled concentration (concentration= 250 g/Nm3), and introduced into the reactor using pneumatic valves (flow rate = 100 sccm)for 60 s and with trapping for an extra 60 s, achieved by closing the outlet valve (small pressure rise during that time). To ensure that the respective precursors were fully removed and to allow IR measurements, the reactor was purged for 10 min with ultrapure N2 gas between each precursor and ozone pulse. The Si substrate was heated to the
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selected deposition temperature and cooled to 100 °C for in-situIR measurements. After each ALD half cycle, an infrared absorption spectrum was recorded using a single-pass transmission geometry. The incidence angle was kept close to the Brewster angle (74˚) to enhance transmission, minimize interference, and provide sensitivity to vibrational absorptions polarized parallel and perpendicular to the surface. Thicker films were grown on 1KÅ-thick thermal SiO2 in a home-built, cross-flow ALD/CVD reactor, which allows the same throughput as in commercial instruments. The reactor chamber was maintained at various temperatures from 200 °C to300 °C. The carrier gas flow for the ALD process was 20 sccm of purified N2from a high-purity liquid nitrogen source. The tributyltin ethoxide precursor was heated to 123–128°C and pulsed by a vapor-draw method. The process flow was a traditional thermal pulse/purge ALD process with a reactor operating pressure of < 1 Torr for both tributyltin ethoxide and ozone. Table S1 in supplementary information summarizes the standard ALD process recipe used for tin oxide deposition using tributyltin ethoxide and ozone in the cross-flow reactor. Note that the geometry requires much shorter ozone exposures, more typical of commercial reactors. However, as indicated below, the process is not fully optimized (complete reactions). Film thickness and optical parameters were measured using a spectroscopic ellipsometry (J. A. Woollam) with the Cauchy model. Resistivity was measured on films grown on 1kÅ-thick thermal SiO2 using a four-pointprobe (Jandel RM3000). XPS measurements are performed using a PHI 5600 XPS spectrometer with monochromatic Al Kα (1486.6 eV) X-ray source (chamber base pressure of 10–10 Torr). Results and Discussion Tributyltin ethoxideand ozone ALD process on OH-terminated SiO2 on Si(111) surface.
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An in-situ FTIR study of the ALD process with tributyltin ethoxide and ozone was performed with the substrate at 200 °C and 300 °C. The starting surface, used as reference for all IR spectra,was a cleaned OH-terminated SiO2-passivated Si(111) surface pre-annealed to 400 °C to isolate the surface hydroxyl groups.15The Figure S2 shows differential absorbance spectrum obtained by referencing the spectrum after the 400 °C pre-annealed to the initial OH-terminated SiO2-passivated Si(111) surface. Figure S2 clearly shows the transformation of H bonded OH groups to isolated OH groups with the broad loss between 3000 cm-1 - 3600 cm-1 with the appearance of 3745 cm-1 after the pre-anneal. Additionally, the native SiO2 densifies after the 400 °C pre-anneal with the gain of TO and LO modes at the lower wavenumber region. The tributyltin ethoxide pulse was 1s, resulting in ~5 mTorr pressure in the ALD reactor, and the ozone pulse was 60s (with an additional 60s trap time at a pressure > 11 Torr). Such long pulse and trap times were used intentional to ensure complete reaction of the ligands in the ALD process.15 Figure 1 shows differential absorbance spectra, i.e. spectra referenced to the spectrum associated with the last step. The top spectrum after the first tributyltin ethoxide exposure(1st) is referenced to the preannealed oxide surface. Then the following spectra (labeled 2nd and 20th) show the changes induced by the tributyltin ethoxide (Sn) or the ozone (O3) pulses, referenced to the surface just prior to this pulse. Upon the first tibutyltin ethoxidepulse (spectrum labeled “1stSn” in Figure 1), there is a clear loss of the band at 3745 cm-1 corresponding to the reaction of isolated surface OH groups, and the appearance of set of bands at ~2900 cm-1 and a band at 1465 cm-1, attributed to the νCH and δCH modes of butyl groups associated with adsorbed precursor molecules. These observations support the expectation that the ethoxide group of tributyltin ethoxide reacts with surface OH groups by releasing an ethanol molecule to form Si-O-Sn bond (~ 1000 cm-1) and that one of the
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butyl groups of tributyltin ethoxide reacts with another surface OH group, releasing a butane molecules to alsoform an Si-O-Sn bond. Upon the first ozone pulse (spectrum labeled “1st O3” in Figure 1), the bands at ~2900 cm-1 and 1465 cm-1disappear (loss in the differential spectrum) indicating that ozone completely reacts with the CH2 and CH3 groups. Initially, some formate species are formed, associated with a relatively strong loss of CH stretch modes. Thereafter, the carbonates bands become visible, keeping in mind that not all the peaks marked “C” have the same intensity. Some peaks are not readily visible in the early cycles but present in the 20th cycles for instance. As for the loss of the CH modes (shown in more detail in Figure S3 in the SI), they exhibit a clear ligand exchange behavior with consistent intensity gain and loss after the first cycle. The weak absorptions that gets intensified after the 2nd ozone pulse appear at 1613 cm-1, 1433 cm-1,1281 cm-1 and 1201 cm-1, assigned to the asymmetric and symmetric stretching bands, νC-OH, and deformation modes, δOH, of carbonate, and at 1574 cm-1and 1362 cm-1, assigned to asymmetric and symmetric stretching COO bands of formate.16, 17 The absorptions at 1241 cm-1 and 1050 cm-1are assigned to C-O stretches from CO2 released during the ozone reaction reabsorbing to form a Sn-O-C-O-Sn functionalization. Since the stretch mode of C=O of carbon dioxide (2350 cm-1), expected if the molecule were physisorbed, is not observed, we postulate that the carbon atom is attached to the surface through two oxygen atoms. Indeed, there is no evidence for the C=O stretch of carboxylate, i.e. no absorption band at 1750 cm-1. Instead, the appearance of a weak band at 3745 cm-1 suggests that hydroxyl groups are formed upon ozone pulse due to a possible Sn-O-Si bond break at the interface. These OH groups are not observed in the latter cycles. Also, during the first two cycles, there is formation of Si-O-Sn bonds, suggesting that it takes two cycles to form the interfacial layer (i.e. fully react with the substrate). Thereafter, we postulate the formation of Sn-O-Sn bonds, although their position is below the spectral sensitivity (
