Atomic Layer Deposition of Silicon Dioxide Using ... - ACS Publications

May 3, 2016 - −1 for. DSBAS, and at 2972, 2185, and 924 cm. −1 for BTBAS] and forming surface Si−O−H (3745 cm. −1. ). ... plasma enhanced co...
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Atomic Layer Deposition of Silicon Dioxide Using Aminosilanes Disec-butylaminosilane and Bis(tert-butylamino)silane with Ozone Luis Fabián Peña,† Charith E. Nanayakkara,† Anupama Mallikarjunan,‡ Haripin Chandra,‡ Manchao Xiao,‡ Xinjian Lei,‡ Ronald M. Pearlstein,‡ Agnes Derecskei-Kovacs,§ and Yves J. Chabal*,† †

Department of Materials Science & Engineering, The University of Texas at Dallas, Richardson, Texas 75080, United Sates Air Products and Chemicals, Inc., 1969 Palomar Oaks Way, Carlsbad, California 92011, United States § Air Products and Chemicals, Inc., 7201 Hamilton Blvd., Allentown, Pennsylvania 18195, United States ‡

S Supporting Information *

ABSTRACT: In situ Fourier transform infrared (FTIR) spectroscopy is used to investigate silicon dioxide deposition on OH-terminated oxidized Si(100) surfaces using two aminosilanes, di-sec-butylaminosilane (DSBAS) and bis(tertbutylamino)silane (BTBAS), with ozone as the coreactant. Both DSBAS and BTBAS readily react at 100 °C with surface −OH groups (loss at 3745 cm−1) with formation of Si−O− SiH3 and Si−O−SiH2−(NHtBu), respectively, through elimination of secondary and primary amines. The (O−)SiH3 structure is characterized by a strong Si−O−Si band at 1140 cm−1, and sharp (O−)SiH3 stretch (2192 cm−1) and deformation (983 cm−1) bands. SiH3 remains stable up to 400 °C, at which point rearrangement into bidentate ((O−)2SiH2) and then tridentate ((O−)3SiH) bonding takes place through condensation reaction with neighboring OH or O groups. In contrast, the O−SiH2−(NHtBu) structure obtained from BTBAS exposure at 100 °C loses its NHtBu group at ∼350 °C, leading to a bidentate bonding ((O−)2SiH2) that remains stable up to 500 °C. In both cases, the transformation to bidentate and tridentate bonding depends on the initial OH concentration. The degree of ligand exchange during atomic layer deposition (ALD) with ozone also depends on the ozone flux. For a high enough flux (≥300 sccm, P ∼ 7.5 Torr), the ligand exchange is essentially complete, with the ozone pulse reacting with Si−Hx [loss of vibrational bands at 2192 and 983 cm−1 for DSBAS, and at 2972, 2185, and 924 cm−1 for BTBAS] and forming surface Si−O−H (3745 cm−1). The initial Si−O−Si band at 1140 cm−1 broadens upon ozone exposure, consistent with the formation of a Si−O−Si network that extends the existing SiO2 substrate. In steady state, the ALD process is characterized by reaction of SiHx by ozone with the formation of OH, thus sustaining the ALD process, with densification of stoichiometric silicon oxide [transverse optical (TO) and longitudinal optical (LO) phonon modes at 1053 and 1226 cm−1].

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INTRODUCTION

particles and chlorine impurities. Additionally, aminosilanes have been shown to be effective for low temperature SiO2 growth using both plasma-enhanced and thermal ALD processes,2,9−11 particularly plasma-enhanced ALD that yields in some cases higher growth rates for SiO2 films than thermal processes using ozone at low temperatures.12 Among the class of aminosilanes used to grow silicon dioxide thin films, two have attracted particular attention because of the high deposition rates they afford at relatively low temperatures (100−400 °C): di-sec-butylaminosilane [(R 2 N)SiH 3 ] (DSBAS)a monosubstituted aminosilaneand bis(tertbutylamino)silane (BTBAS)a disubstituted aminosilane (Figure S1 in the Supporting Information). DSBAS exhibits a

Silicon dioxide is arguably the most important dielectric material for the semiconductor and microelectronic industry.1 With the development of complex, three-dimensional (3-D) structures in advanced devices, there is a need to deposit ultrathin and conformal SiO2 films.2 Atomic layer deposition (ALD) is the most attractive method to achieve highly conformal, uniform, and reproducible film growth.3 It is therefore not surprising that a number of experimental and theoretical ALD studies have been performed in recent years, using thermal and plasma-enhanced processes, to deposit silicon dioxide.4−7 Different types of silicon precursors and oxidizing agents have been used as the chemical structures of the precursors have been shown to play an important role in the ALD growth of SiO2.8 Among such precursors, aminosilanes are attractive because they do not produce reactive byproducts often generated by other silicon precursors, or © XXXX American Chemical Society

Received: February 22, 2016 Revised: April 29, 2016

A

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The Journal of Physical Chemistry C higher film growth rate per cycle (GRPC) under thermal and plasma enhanced conditions at low temperatures (150−300 °C) than its disubstituted counterpart BTBAS.2 Yet, despite its lower GRPC compared to DSBAS, BTBAS has been shown to have a higher GRPC than other amine-based silicon precursors, such as bis(ethylmethylamino)silane (BEMAS) and bis(diethylamino)silane (BDEAS), at temperatures between 250 and 300 °C.2 With ozone as the oxidizer, the reported GRPC for DSBAS is ∼1.6 Å/cycle at 150 °C.2 Mechanisms for DSBAS and BTBAS adsorption on silicon oxide and subsequent ALD reaction with ozone have been proposed, primarily on the basis of ab initio (density functional theory) calculations.13,14 The calculations address both the initial chemisorption of the aminosilane molecules on OHterminated silicon oxide and the ALD process in which ozone reacts with the adsorbed silicon precursor. Both DSBAS and BTBAS are shown to react with surface OH groups through their amino ligand (elimination of secondary and primary amines, respectively) with formation of surface Si−O−SiH3 and Si−O−SiH2−(NHtBu) products, respectively. In addition, theoretical modeling suggests that condensation reactions with neighboring unreacted surface OH groups are favorable and could transform (O−)SiH3 species into (O−)2SiH2 or (O)−SiH2−(NHtBu) into (O−)2SiH2.13,14 For the ALD process, calculations find that ozone directly attacks Si−H bonds by oxygen insertion, converting initially adsorbed (O−)SiH3 into (O−)2Si−(OH)2 and initially absorbed −SiH2−(NHtBu) into (O−)2Si−(OH)2. A spectroscopic study of the complete process is particularly important because the effects of several parameters, such as initial OH concentration, surface roughness, and ozone flux, are not considered in theoretical models, yet are critical to applications. For example, variations in ozone flux can lead to qualitatively different surface species,15 which can change the surface reactivity for subsequent ALD processes. In this work, in situ Fourier transform infrared (FTIR) spectroscopy is used to study the surface reactions during the initial stages of silicon dioxide ALD thin film growth using DSBAS and BTBAS at low deposition temperatures. Furthermore, quantum chemical calculations are performed to determine the stable adsorbed species, their respective vibrations, and the reaction pathways leading to these structures.

These clean samples are introduced through a load-lock into the N2 purged ALD reactor. In the ALD reactor, the DSBAS and BTBAS precursors are vapor drawn from an ampule kept at 50 °C and pulsed into the reactor (typically 5−10 s pulses). Ozone (O3) is produced using an In-USA ozone generator with controlled concentration (13−18 wt % O3/cm3), and introduced into the reactor using pneumatic valves, with exposure times and flow rates ranging from 0 to 60 s pulses and from 100 to 300 sccm, respectively. For ALD growth, the sample is exposed to alternative pulses of DSBAS (or BTBAS) and ozone followed by 15 min purging with ultrapure N2 gas between precursor pulses. The temperature of the Si substrate is maintained at 100 °C during the ALD growth and cooled to 80 °C for all in situ IR measurements. After each ALD half-cycle, an infrared absorption spectrum is recorded using a single-pass transmission geometry. The incidence angle is 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.

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FIRST-PRINCIPLES CALCULATIONS Density functional theory (DFT) calculations are performed using Dmol3 as implemented in Materials Studio 6.0 by Biovia using the BLYP density functional with the DNP basis set in the all electron approximation.17,18 The SiO2 surfaces with the various surface groups are modeled with a periodic slab composed of five layers of silicon with OH termination at the top and bottom of the slab to avoid unsaturated valencies. The bottom four layers are constrained during geometry optimization. The slab model is prepared by cleaving crystalline βcristobalite along the 001 Miller plane (Figure S3.)

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RESULTS AND DISCUSSION

Initial SiO2 Surface, Surface Hydroxyl Density, and Initial Reaction of DSBAS. The initial surface OH concentration and structure are important factors for the subsequent ALD process, starting with the reaction of the silicon precursors. To a great extent, the vibrational spectrum associated with surface OH groups provides most of the relevant information. The O−H stretching mode can be observed over a wide frequency range (3200−3750 cm−1), due to the fact that hydrogen bonding greatly red shifts the vibration.19 McMillan et al. showed that the high-frequency edge of the asymmetric peak of corresponding SiOH groups are those that are not involved in any hydrogen bonding, or are essentially “free”, and that the low-frequency tail of the OH stretch band reflects SiOH groups subject to varying degrees of hydrogen bonding.20 To highlight the importance of the OH arrangement, Figure 1 focuses on the 4000−3000 cm−1 region, from which the reaction of DSBAS with surface OH and the formation of OH upon ozone exposure during the first three ALD cycles can be inferred. The top spectrum in Figure 1 shows that DSBAS reacts mostly with isolated OH (loss of a sharp peak centered at 3740 cm−1). After the first ozone exposure, there are some OH species generated that involve some H-bonding as the spectrum is broadened toward the lower frequencies. The schematic diagram in Figure 1 illustrates the mixture of isolated (∼3740 cm−1) and hydrogen-bonded (red shifted from 3740 cm−1) OH groups that account for the observed spectra. Quantification is difficult because the O−H stretch absorption is enhanced by H-

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MATERIALS AND METHODS For this study, a home-built ALD reactor designed for in situ IR absorption measurements is utilized.16 Infrared absorption spectra are recorded with a Thermo Nicolet 6700 spectrometer (frequency range of KBr beamsplitter, 400−4000 cm−1) equipped with a mercury cadmium telluride (MCT/B) detector. A pneumatically controlled sliding shield protects the KBr windows during precursor exposures. The 0.05 cm thick Si(100) samples are cut into 3.8 × 1.5 cm2 pieces from a double-sided polished, float-zone-grown, thermally oxidized and lightly doped (ρ ∼ 10 Ω cm) wafer. Samples are then subjected to a three-step degreasing sequence consisting of sequential 5 min long immersions in dichloromethane, acetone, and methanol. The thin thermal oxide (∼6 nm) is further cleaned in a piranha solution consisting of 1:3 H2O2:H2SO4 at 80 °C for 30 min, then thoroughly rinsed in deionized water, and blown-dry with N2 gas. At this point, the resulting clean oxide is terminated with a high concentration of surface hydroxyl groups that are close enough for extensive H-bonding. B

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adsorb in bi- and tridentate configurations and not all Si−H species are reacted. There is, however, a way to control the initial OH concentration on the surface by optimizing the concentration of isolated OH groups, which is helpful to identify and quantify surface reactions. This can be achieved by annealing the chemically cleaned SiO2 surface to ∼500 °C in a flow of dry N2 gas. Such preannealing fosters the condensation reaction of Hbonded Si−OH groups that are in close proximity (i.e., Hbonded), releasing H2O and forming Si−O−Si bonds, and leaving all remaining OH groups isolated, i.e. not H bonded. The corresponding IR spectra are displayed in Figure 2a, in which the initial, cleaned SiO2 surface is used as reference. Upon annealing, there is a broad loss in the 3725−3125 cm−1 region, associated with condensation reaction of a fraction of the surface OH (net loss of OH stretch intensity), and the appearance of isolated OH (centered at 3745 cm−1). We have also determined that the resulting isolated OH groups remain stable to temperatures as high as 850 °C, in contrast to Hbonded OH groups. To quantify the amount of isolated OH groups on the annealed SiO2 surfaces, we can use a model system that has a very well defined OH concentration, namely the clean Si(100)(2×1) surface after full H2O reaction, probed by IR in the same configuration.21 After water reaction, half of the surface (one end of each dimer) supports OH groups (3.4 × 1014 /cm2),22 setting the OH absorbance at 0.0033 cm−1/monolayer (ML). With this calibration and the values summarized in Table 1 for the OH stretch integrated areas, we estimate the density of OH groups after 500 °C annealing to be ∼2.2 × 1014/cm2. This information makes it possible to quantify the density of SiH3 groups after DSBAS reaction (Figure 2b), as summarized in Table 1. A comparison of the integrated areas in Table 1 (second column) shows that preannealing to 300 and 500 °C increases the concentration of isolated OH, resulting from a reduction of hydrogen-bonded OH (broad band) obtained from Figure 2a. After reaction with DSBAS, the loss in the OH-stretch region (from Figure 1) provides an estimation of “reacted OH” and

Figure 1. Differential infrared absorbance spectra in the O−H stretch region after DSBAS/ozone ALD cycles on SiO2 on Si(100), referenced to previous treatment. Processing conditions: 5 s DSBAS exposure (P = 1200 mTorr) and 60 s ozone exposure (P = 7.5 Torr) with 100 °C substrate temperature.

bonding. However, the total Si−Hx stretch integrated area after ozone exposure (0.0167 absorbance·cm−1) is clearly larger than the total area loss in the top spectrum (0.0145 absorbance· cm−1) (Figure S4), which is consistent with oxidation of SiH3, as there can be up to 3 times more OH formed from adsorbed SiH3 than original reacted OH sites. During subsequent ALD cycles, the OH stretch spectrum remains essentially broad, with even more red-shifted contributions. This is consistent with the formation of constrained OH groups on an amorphous surface. The positive and negative contributions are similar in magnitude, consistent with a steady state ligand exchange. Since the Si precursor has the potential to triple the number of OH bonds at each cycle, which is clearly not sustainable, a steady state is reached wherein the initial precursor tends to

Figure 2. (a) Infrared absorbance spectra of a cleaned OH-terminated SiO2 on Si(100) sample in the 3000−4000 cm−1 region after 300 and 500 °C annealing, referenced to the starting clean surface, all measured with the sample at 80 °C. (b) Infrared spectra of these same samples (clean starting surface, annealed to 300 °C, and annealed to 500 °C) after DSBAS exposure, referencing all spectra after DSBAS exposure to the spectra before the DSBAS exposure (all spectra collected at 80 °C). Processing conditions: 5 s DSBAS exposure (P = 1200 mTorr) and 100 °C substrate temperature. C

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Table 1. Integrated Areas between 3800 and 3700 cm−1 of Vibrational Stretch of Isolated OH Formed by Annealing and Reacted after DSBAS Exposurea,b preanneal T (°C)

initial isolated −OH (gain at 3740 cm−1)

reacted −OH (loss of broad band)

estd (O−)SiH3 (2000−2300 cm−1)

300 500

0.0026 ± 0.0001 0.0021 ± 0.0001

0.0037 ± 0.0001 0.0027 ± 0.0001

0.0172 ± 0.0002 0.0094 ± 0.0002

a Units = absorbance·cm−1 (error bars are for area determination, not reproducibility). bThe integrated areas of O−SiH3 formed by DSBAS reaction between 2300 and 2000 cm−1 are also included for the comparison.

Figure 3. (a) Infrared absorbance spectra of OH-terminated SiO2 Si(100) after DSBAS exposure referenced to the surface with a 500 °C preanneal treatment. (b, c, d, and e) Differential infrared absorbance spectra during the stepwise annealing to 200, 300, 400, and 500 °C, each one referenced to the spectrum corresponding to the previous annealing temperature (all spectra recorded with the sample at 80 °C).

DSBAS Dissociative Chemisorption and Thermal Stability of Adsorbed Species. Figure 3a displays the complete spectrum obtained after DSBAS exposure with the substrate at 100 °C, referenced to the preannealed SiO2 surface. In addition to the loss of OH at 3745 cm−1 and the appearance of the SiH3 stretch mode at 2192 cm−1, three new features are observed at 1140 cm−1, 981 cm−1, and a shoulder at 966 cm−1. The high frequency band was previously assigned to the oxygen vibration of the Si−O−SiH3 species. The two lower frequency bands are assigned by first-principles calculations (Table 2) to the SiHx deformation modes (umbrella and scissor) of (O−)SiH3 and (O−)2SiH2, respectively. There is no evidence for adsorbed CHx bands (∼2900 cm−1), suggesting that the reaction is complete and there are no physisorbed DSBAS

the integrated area comparison shows that more OH was reacted than the amount of purely isolated OH formed by preannealing. This confirms that there was still some H-bonded OH remaining after annealing (300 and 500 °C) and that such OHs can also react with DSBAS. Such H-bonded OH is hard to detect (Figure 1) because the band is very broad (red-shifted) and therefore hard to observe in the differential spectra in Figure 1 (top spectrum). Note that the error bar listed in Table 1 only reflects the precision in area measurement (integrated area), not the reproducibility of the measurements. Therefore, the overall message is that the amount of H-bonded OH remaining at the surface decreases with increasing preanneal temperature, as estimated by subtracting the area of the “initial isolated OH” from the “reacted OH”: the areas of H-bonded OHs lost are 0.0011 and 0.0006 for 300 and 500 °C, respectively. Consequently, the resulting coverage of (O−)SiH3 is higher after DSBAS exposure on the surface annealed at 300 °C than on the surface annealed at 500 °C, with a ratio (∼1.8) higher than just the ratio of the isolated OH groups (∼1.2). As shown in the section DSBAS Dissociative Chemisorption and Thermal Stability of Adsorbed Species, the stability of adsorbed SiH3 on the surface (i.e., of the (O−)SiH3 species) is also dependent on the initial OH density at the surface. With a higher starting OH density, the adsorbed SiH3 is observed to condense with neighboring unreacted OH to form (O−)2SiH2, as evidenced by a broadening of the Si−O−Si mode at 1148 cm−1.

Table 2. Assignments of the Measured IR Vibrational Bands (in cm−1) from Calculateda and Literature Values exptl Si−OH (O−)SiH3 (O−)2SiH2 (O−)3SiH

a

D

3745 2192 ± 4 983 ± 4 2217 ± 3 966 2269 ± 4 n.o.b 882 ± 3

calcda 2220, 2194 989, 985 2257 997, 988 2297 969, 953 917

lit.

ref

3740 2136, 2140 863 2200 940 2247, 2250

23 24, 25 24 26 25 9, 26

880

26

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Figure 4. Differential infrared absorbance spectra during 1−12 ALD cycles with DSBAS (nth DSBAS) and ozone (nth ozone) (flux = 200 g/Nm3, 1 min exposure with trapping) on OH-terminated SiO2 on Si(100), referenced to each preceding treatment.

molecules. The presence of a shoulder at 966 cm−1 indicates that a small amount of (O−)2SiH2 has been formed through condensation. Note that the Si−H2 stretching mode of the (O−)2SiH2 moiety, expected at 2220 cm−1, is not clearly resolved after DSBAS exposure in the Figure 3a. Two bands at 953 and 969 cm−1, associated with Si−O modes, are not detectable experimentally, possibly due to their interaction with substrate phonon modes or with the Si−H bending mode. In fact, this may be the reason why only one mode is detected instead of three for SiH in (O−)3Si−H, as the other two vibrations involve a combination of Si−O and Si−H vibrations. The mechanism for dissociative chemisorption of DSBAS, previously proposed by Huang et al.,14 involves Si−N bond breaking in DSBAS and reaction with surface Si−O−H groups, via H transfer to the amine group and elimination of HNR2 (R = secondary butyl group), leaving SiH3 on the silicon surface. Our findings for the reacted and adsorbed species are consistent with this mechanism. Thermal Stability of Adsorbed DSBAS. Structural modifications of adsorbed SiH3 are examined in situ by IR spectroscopy upon annealing under N2 purge (Figure 3b−e). At 300 °C, there appears to be a conversion from (O−)SiH3 to (O−)2SiH2 bonding, as evidenced by the appearance of the SiH2 stretching band at 2220 (Figure 3c). At 400 °C, (O−)3SiH begins to be formed, with characteristic IR stretching (2280 cm−1) and deformation (883 cm−1) bands. The calculations indicate that there is a reaction (kinetic) barrier for this (O−)SiH3 to (O−)2SiH2 interconversion, due to the (O−)SiH3 reacting with a neighboring unreacted −OH groups resulting in (O−)2SiH2 and elimination of an H2 molecule, as previously noted by Huang et al.14 Therefore, annealing to 300 °C may help to overcome the energy barrier and lead to SiH2 formation, and annealing to 400 °C is required to continue the reaction of (O−)2SiH2 with another adjacent Si−OH group to form (O−)3SiH with the elimination of another H2 molecule.27

ALD Process with DSBAS and O3. The differential IR spectra of the 1st, 2nd, 11th, and 12th ALD cycles of the DSBAS/ozone are presented in Figure 4. The spectrum of the first DSBAS pulse (“1st DSBAS”) on the hydroxylated SiO2 surface shows the loss of the characteristic signature of −OH groups at 3745 cm−1 (negative peak) and the appearance of the O−SiH3 vibrational modes (positive peaks) at 2192 cm−1 for stretching and 983 cm−1 deformation and of a band at 1140 cm−1 (Si−O−Si stretching), consistent with a complete reaction. For the next step in the ALD process (O3 treatment), control of the flux is important because the ozone flux can alter the reaction products on the surface.15 We address this point by starting the experiment with an average ozone flux (200 g/Nm3 with 100 sccm flow rate), which we will show is not quite sufficient to fully react SiH3 (see the section Dependence on Ozone Exposure). Under these conditions, the effective ozone pressure in the reactor is 4.8 Torr (with ozone trapping) during the 60 s pulse. The complete removal of SiH3-related modes and the appearance of a band at 3745 cm−1 (positive peak) indicate that ozone reacts by oxygen insertion to form Si−OH groups. Concurrently, two bands appear at 1053 and 1226 cm−1 after each ozone pulse, assigned to the transverse optical (TO) and longitudinal optical (LO) phonon modes of SiO2, indicating that Si−O−Si bonds are formed on the SiO2 substrate upon ozone treatment. The rest of the differential spectra in Figure 4 show that, for the first 12 cycles, there is a ligand exchange process (appearance and removal of ligand or product modes), which is typical of a standard ALD mechanism. Therefore, our experimental data are consistent with a well-behaved ALD process with DSBAS and ozone at 100 °C under the conditions specified above. Additionally, the appearance of weak bands at 2269 cm−1 (stretching) and 879 cm−1 (deformation) indicates that a small amount of (O−)3SiH is formed after the first ozone pulse, which could be a result of incomplete SiH3 reaction with ozone. The amount of (O−)3SiH is further increased after the second DSBAS exposure. This could be due to the proximity of the some of the −OH groups formed during the previous E

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Figure 5. Infrared absorbance spectra during 1−12 ALD cycles with DSBAS (nth DSBAS) and ozone (nth ozone) (flux = 200 g/Nm3, 1 min exposure with trapping) on OH-terminated SiO2 on Si(100), referenced to the spectrum after a 500 °C preanneal treatment prior to ALD.

Figure 6. Differential infrared absorbance spectra during 1−3 ALD cycles with DSBAS (nth DSBAS) and ozone (nth ozone) (flux = 280 g/Nm3, 2 min exposure) on OH-terminated SiO2 on Si(100), referenced to each preceding treatment.

ozone exposure, facilitating (O−)SiH3 → (O−)2SiH2 → (O−) 3SiH condensation by H 2 elimination after each conversion. After the next ozone pulse (“2nd Ozone”), there is some removal of bands associated with (O−)3SiH but not completely, possibly due to the higher stability of this Si−H species or lower accessibility of ozone to their location. For this reason (O−)3SiH remains a stable byproduct during the ALD cycles, leading to H accumulation in the film, as detailed below. Although differential spectra (Figures 3 and 4) are most helpful to examine changes taking place after each half-cycle (e.g., ligand exchange), the best way to identify possible accumulation of species in the film during ALD is to plot the

spectra collected after each step, referenced to the initial preannealed surface prior to any ALD pulse. The results are shown in Figure 5 (for DSBAS) after each half-cycle of the 1st, 2nd, 11th, and 12th cycles. Except for the loss of the initial surface OH on SiO2, all absorption bands are positive, indicating the formation and retention of some species. These include the TO and LO phonon modes of deposited SiO2 film at 1226 and 1053 cm−1, which can be clearly seen to increase with the number of cycles. In contrast to the absorption bands associated with (O−)SiH3 at 2192, 983, and 965 cm−1 that appear after the DSBAS exposures and are removed (no peaks) after ozone exposure, the bands associated F

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Figure 7. Differential infrared absorbance spectra during 1−12 ALD cycles with BTBAS (nth BTBAS) and ozone (nth ozone) (flux) on OHterminated SiO2 on Si(100), referenced to each preceding treatment.

Figure 8. Infrared absorbance spectra during 1−12 ALD cycles with BTBAS (nth BTBAS) and ozone (nth ozone) (flux) on OH-terminated SiO2 on Si(100), referenced to the spectrum after 500 °C preanneal treatment. Note that the cumulative loss at ∼2350 cm−1 is due to an overall reduction of CO2 gas in the spectrometer due to purge variation, i.e. is not associated with gas in the ALD reactor.

with (O−)3SiH at 2269 and 879 cm−1 remain visible and intensify with the number of cycles. The persistence of these species suggests that the ligand exchange is not complete. Dependence on Ozone Exposure. We now show that increasing the ozone flux to 280 g/Nm3 and 300 sccm flow (2 min exposure) leads to almost complete removal of the SiH3 vibrational bands at 2269 and 879 cm−1, indicating a full ALD reaction (Figure 6). The remaining Si−H observed near 2269 cm−1 with lower ozone flux is now almost completely removed. The bend at 883 cm−1 is also much weaker after DSBAS exposure, which is consistent with reaction of the Si−H bonds with increasing the ozone flux and flow rate (Figure S2). These

observations indicate that Si−Hx reactivity is dependent on the ozone exposure. The current ozone parameters have been optimized to maximize reaction with SiHx species. BTBAS Dissociative Chemisorption. The reaction of BTBAS with −OH-terminated surfaces associated with SiO2 on Si(100) has been previously proposed by Han et al. as dissociative chemisorption of the BTBAS molecule resulting in O−SiH2−(NHtBu) surface species and leaving the tertbutylamine molecule.13 Figure 7 shows the ALD process for BTBAS and ozone at 100 °C for 12 cycles, using 300 sccm ozone flow rate and 280 g/Nm3 ozone concentration. As shown in Figure 7 (“1st BTBAS”), the first BTBAS pulse reacts with G

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The Journal of Physical Chemistry C the surface as evidenced by the loss of −OH groups at 3745 cm−1 (negative peak) and the appearance of bands at 2185 cm−1 (stretching) and 924 cm−1 (deformation) associated with SiH2 in the O−SiH2−(NHtBu) species (positive peaks), consistent with dissociative chemisorption of the precursor at the surface. Furthermore, all the bands associated with the BTBAS ligands after chemisorption are detected, such as the −CHx stretching at ∼2900 cm−1, C−N stretching at 1230 cm−1, Si−N stretching at 1026 cm−1, and CH3 bending at 1380 cm−1. ALD Process with BTBAS and O3. When the high ozone flux conditions are used (300 sccm flow rate with 280 g/Nm3 ozone concentration), the subsequent ozone pulse completely removes all characteristic bands associated with the O−SiH2− (NHtBu) surface complex and produces a feature at 3740 cm−1 (positive peak) indicating the formation of −OH groups (Figure 7). Additionally, the SiO2 TO and LO phonon bands are observed at 1250 and 1070 cm−1, confirming the growth of SiO2 after each ozone pulse (Figures 7 and 8). Two other bands are observed at 1656 and 1300 cm−1 following the ozone exposure, tentatively assigned to carbonate and/or formate species. These have been previously observed when ozone is used as the coreactor for oxide growth using ALD,23 and are not fully removed by BTBAS. Otherwise, the steady-state ALD process is characterized by ligand exchange with the removal of characteristic bands for O−SiH2−(NHtBu) and the appearance of hydroxyl groups upon ozone exposure, together with evidence for a more stoichiometric and denser silicon oxide (TO and LO phonon modes at 1250 and 1070 cm−1). For comparison with DSBAS (Figure 5), Figure 8 focuses on the spectral regions of the NHtBu ligand and SiO2 vibrations for the 1st, 2nd, 11th, and 12th BTBAS/ozone ALD cycles referenced to the spectrum obtained after the 500 °C preanneal. The spectra confirm the growth of SiO 2 , characterized by strong and distinct TO/LO modes at 1250 and 1070 cm−1, and the complete removal of the NHtBu ligand, although the thickness of SiO2 is less than with DSBAS as discussed next. A quantitative comparison of SiO2 film growth is obtained by using the integrated peak areas of TO−LO phonon modes of SiO2 for both DSBAS/ozone (Figure 5) and BTBAS/ozone (Figure 8) depositions after depositing 3 and/or 12 cycles. The calibration of the integrated IR absorbance is obtained independently by ellipsometry using a 12-nm-thick SiO2 on Si(100) standard. The results are summarized in Table 3. For comparison, the deposition achieved for DSBAS using a low ozone flux is also included. The data clearly show the important role of the ozone flux. Higher fluxes lead to more complete ligand exchange (see Figure 6 for ozone pulses of 280 g/Nm3 and 300 sccm, 2 min exposure) and higher deposition rates. Therefore, it is critical to use an ozone flux that can fully remove the precursor ligand during SiO2 ALD deposition. In our case, it was not possible with the current generator and reactor geometry to go higher than 280 g/Nm3 and 300 sccm, but this value appeared to be sufficient for a complete ALD process. Table 3 clearly shows that a higher SiO2 growth rate is achieved with DSBAS than with BTBAS for the same ozone flux. The thicknesses determined here are comparable to previously reported thicknesses using commercial ALD reactors with the same precursors. In this work, the maximum number of cycles is only 12, which makes the estimate of the growth rate per cycle more uncertain than obtained from hundreds of

Table 3. Calculated SiO2 Growth per Cycle for DSBAS/ Ozone and BTBAS/Ozone Estimated from Integrating the TO and LO Mode Intensity of SiO2 Deposited after 3 and/ or 12 ALD Cycles and Calibrating It with a Sample with Known Thickness Measured by Ellipsometrya ozone concentrationb

DSBAS (Å/cycle)

BTBAS (Å/cycle)

3 cycles

3 cycles

12 cycles

1.5

1.8

3

200 g/Nm and 100 sccm (1 min exposure), Pozone = 1.2 Torr 200 g/Nm3 and 100 sccm (1 min exposure with trapping), Pozone = 4.8 Torr 280 g/Nm3 and 300 sccm (2 min exposure), Pozone = 7.5 Torr

12 cycles

1.0 1.8 2.4

a

The aminosilane precursor pulses are 5 s long, producing a pressure of ∼1.2 Torr. The error associated with the estimation of the thickness per cycle is ±0.1 Å. bg/Nm3 = grams of ozone per normal cubic meter.

cycles in commercial ALD reactors; nonetheless it is a good estimate for the calculated GRPC and confirms that the ozone flux conditions are sufficient.

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CONCLUSIONS DSBAS and BTBAS precursors are shown to be suitable for atomic layer deposition of SiO2 thin films on −OH-terminated Si(100) surfaces using ozone as a coreactant. The observed in situ IR absorption spectra confirm the previously proposed reaction mechanisms for both aminosilane precursors, but show that (O−)3SiH is also formed, which had not been taken into account in the mechanism. The results indicate that the substrate temperature and the ozone flux both affect the surface termination after the first half-cycle, i.e. aminosilane pulse. For instance, while deposition at 100 °C leads to the formation of (O−)SiH3 for DSBAS, deposition at higher temperatures (>300 °C) leads to a transformation into (O−)2SiH2, as condensation of (O−)SiH3 with a neighboring unreacted OH takes place, and into (O−)3SiH at 500 °C, which is the most stable hydride species. Furthermore, a minimum ozone flow of 280 g/Nm3 and 300 sccm is necessary for a full ALD process (complete ligand exchange) for the reactor used. With this ozone flow, a substrate temperature of 100 °C is adequate for a complete ALD process with DSBAS. Similar conclusions are obtained for BTBAS, although the growth per cycle is slightly lower and there is some incorporation of formates/carbonates in the interfacial region.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b01803. Ball-and-stick model of aminosilane precursors (Figure S1); differential absorption spectra of different ozone flux exposure reactions with DSBAS (Figure S2); schematic drawing of the slab model used for DFT calculation of vibrational frequencies (Figure S3); differential absorption spectra of Si−Hx stretch integrated area after ozone exposure (Figure S4) (DOCX)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. H

DOI: 10.1021/acs.jpcc.6b01803 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Air Products and Chemicals and the National Science Foundation (Grant CHE-1300180).

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DOI: 10.1021/acs.jpcc.6b01803 J. Phys. Chem. C XXXX, XXX, XXX−XXX