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In Situ Reaction Mechanism Studies on Lithium Hexadimethyldisilazide and Ozone Atomic Layer Deposition Process for Lithium Silicate Yoann Tomczak,* Kjell Knapas, Markku Sundberg, Markku Leskela,̈ and Mikko Ritala Laboratory of Inorganic Chemistry, University of Helsinki, P.O. Box 55 FIN-00014 University of Helsinki, Finland ABSTRACT: Reaction mechanisms in the LiN[Si(CH3)3]2−O3 atomic layer deposition (ALD) process for lithium silicate were investigated in situ with a quartz crystal microbalance (QCM) and a quadrupole mass spectrometer (QMS) at several temperatures. In addition, ex situ Fourier transform infrared (FT-IR) measurements were carried out to identify the bonds present in the films. QMS indicates typical combustion byproducts such as CO2 (m/z = 44), CO (m/z = 28), H2O (m/z = 18) and NO (m/z = 30) during the ozone pulse. Signals corresponding to the fragments of the ligands are present, but their low intensities imply that there are no direct ligand exchange reactions with the hydroxyl groups on the surface. QCM results confirm the decomposition of the ligand through complex reactions upon reaching the surface. Accordingly, several reaction pathways were drawn, and DFT calculations were performed to assess the reactivity of each reaction intermediate. The influence of the deposition temperature on several characteristics of the process such as composition of the film and growth per cycle was also explained.

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INTRODUCTION Atomic layer deposition (ALD) is a method of choice to deposit thin films with excellent uniformity and conformality.1−3 In ALD, the precursors are alternately pulsed into the reactor and separated by purging with inert carrier gas. The surface is successively saturated with monolayers of adsorbed or reacted precursor molecules, hence providing a self-limiting growth mechanism. Therefore, the thickness of the growing film is accurately controlled by the number of ALD cycles repeated. The reactions occurring at each step of the ALD cycle are often complex. Studies on such mechanisms help to control and optimize the growth processes and accelerate the development of new ALD processes and precursors. In situ analytical methods have now become common for the investigation of ALD reaction mechanisms. Several techniques such as Fourier transform infrared (FT-IR),4−6 ellipsometry,7−9 X-ray reflectometry (XRR), X-ray photoelectron spectroscopy (XPS)10,11 and X-ray fluorescence (XRF)12 have been employed to provide valuable information on the ALD processes. The most common in situ techniques in analyzing ALD chemistry have, however, been quartz crystal microbalance (QCM) and quadrupole mass spectrometry (QMS), which are used also in this study. The QCM indicates the mass changes of the growing film, while the QMS follows the composition of the gas phase inside the reaction chamber. The identification of the gaseous byproducts and the recorded mass changes at each step of the ALD cycle allows us to draw a precise picture of the overall reaction mechanism.13−20 Lithium silicates can be used in lithium ion batteries as solid state electrolytes, leading to all solid state batteries with © XXXX American Chemical Society

improved efficiency and safety. Amorphous Li2SiO3 possesses a higher ionic conductivity compared to crystalline forms of lithium silicates.21,22 Its ionic conductivity can be further increased by tailoring the film thickness and the substrate material. A mixture of amorphous Li2SiO3 and Li2Si2O5 was also reported for ozone gas sensing.23 Lithium hexamethyldisilylazide (LiN[Si(CH3)3]2, LiHMDS), also known as lithium bis(trimethylsilyl)amide, is a compound widely used in synthesis of other inorganic precursors24 but our group recently demonstrated that LiHMDS can also be used with ozone for the ALD of amorphous Li2SiO3:25 The LiHMDS precursor showed self-limiting surface reactions, and the resulting films had good conformality and uniformity with a precise control over the thickness. However, the growth rate and the composition of the deposited film seem to be highly dependent on the reaction temperature. ALD of lithium nitride and lithium carbonate from LiHMDS was also investigated recently.26,27 In the present study, the reaction mechanism of the LiHMDS−O3 ALD process is explored using in situ QCM and QMS analysis combined with ex situ FT-IR studies and DFT calculations.

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EXPERIMENTAL SECTION The experiments were performed with a commercial flow-type F-120 ALD reactor manufactured by ASM Microchemistry Ltd. specially modified28 for the in situ measurements. The Received: December 14, 2012 Revised: June 3, 2013

A

dx.doi.org/10.1021/jp312309g | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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substrates used are made of soda lime glass with a total area of about 3500 cm2. Argon (Oy AGA Ab, 99,999%) was used as the carrier and purging gas with about 3 mbar pressure inside the reactor. The gas species present in the reactor during the ALD cycle were investigated using a Hiden HAL/3F 501 RC QMS with a Faraday cup detector and an ionization energy of 70 eV. The pressure in the QMS chamber was around 1 × 10−5 mbar, obtained by differential pumping through a 100 μm orifice. The mass changes on the substrate were recorded using a Maxtek TM 400 QCM with a sampling rate of 20 Hz. The lithium hexamethyldisilylazide (LiHMDS) was obtained from Sigma-Aldrich. It was held inside the reactor in an open source boat at 65 °C and pulsed using inert gas valving.2 During the experiments, the temperature of the substrates was 200, 250, or 300 °C. Ozone was produced from 99.999% O2 (Oy AGA Ab) with an ozone generator (Wedeco Ozomatic modular 4 HC Lab Ozone) and pulsed into the reactor with solenoid and needle valves. The ozone concentration was 45 g/Nm3. While analyzing the QMS results, the background arising from the fragmentation of the precursors had to be evaluated and subtracted from the QMS data measured during the ALD cycles: Five reference pulses were applied to assess the contribution of the reactants to the integrated intensities measured for a specific m/z signal. The reference pulses were launched before the ALD cycles for the oxygen precursor and after the ALD cycles for the metal precursor. Accordingly, the following pulsing sequence was used for the LiHMDS−O3 process: - Ozone reference pulses: 5× (10 s O3 pulse/100 s purge) - ALD cycles: 10× (20 s LiHMDS pulse/20 s purge/10 s O3 pulse/60 s purge) - LiHMDS reference pulses: 5× (20 s LiHMDS pulse/90 s purge) Samples for FT-IR spectroscopy were grown on a Si(100) substrate with 1000 cycles of the LiHMDS−O3 process (pulse/ purge lengths: 4s/2s/2s/1s). Absorbance spectra were recorded with a FT-IR Spectrum GX from Perkin-Elmer, each consisting of 100 scans over the 400−4000 cm−1 interval. The resolution was 4 cm−1. The spectrum of the Si(100) substrate was measured as a reference, and its contributions were subtracted from the sample spectra. Geometry of the molecules potentially involved in the adsorption of LiHMDS was studied by computational methods at the B3LYP/6-311g* level. Frequencies for the optimized structures were computed to know whether the structures corresponded to genuine energy minimum. All possible conformers for each compound were optimized, and the one possessing the lowest energy was then studied further. Atomic charges were computed by NBO methods included in the Gaussian suite.39

Figure 1. FT-IR spectra of a 80 nm thick thin film deposited with the LiHMDS−O3 process.

attributed to Li2CO330 formed by reaction of lithium species with CO2 either during the growth or afterward upon exposure to air. The weak band at 1300 cm−1 indicates the presence of Si-CH3 bonds in the film, originating from incomplete combustion of the −Si(CH3)3 moiety by ozone. Table 1 displays the signals investigated with QMS and the corresponding fragments. During the ozone pulse, byproducts Table 1. QMS Signals and Their Corresponding Fragments m/z 15 17 18 30 31 43 44 65,5 73 74 130 146 147

corresponding fragments

signal detected

byproducts during LiHMDS pulse

byproducts during O3 pulse

CH3+ NH3 H2O+ NO, C2H6+ CH2(OH)+ HSiCH2+ CO2+ HN(SiMe2)22+ SiMe3+ HSiMe3+ N(SiMe2)2+ N(SiMe3)2+ −CH3 HN(SiMe3)2+ −CH3

yes yes yes yes yes yes yes no no no no no

no yes no no no no no no no no no no

yes no yes yes yes yes yes no no no no no

no

no

no

were observed at m/z = 44, m/z = 30, and m/z = 18, which respectively correspond to CO2, NO, and H2O, typical combustion byproducts in the ozone based ALD oxide processes.17,31,32 No signs of silicon containing byproducts were detected during either pulse, but signals originating from fragments of the ligand (m/z = 146, 73, 65.5) were observed during the precursor pulses. However, their intensity was slightly lower during the ALD cycles than during the background pulses (Figure 2). This implies an adsorption of the precursor on the surface without direct exchange of its ligand with −OH groups on the surface: the LiHMDS molecule adsorbs through a more complex mechanism to be detailed further. Figure 3 shows the QCM mass change for a full cycle of the LiHMDS-O3 process: First, the mass increases during the LiHMDS pulse, traducing adsorption of the precursor molecules on the surface. During the following purge, the mass stabilizes after a slight decrease due to desorption of some byproduct molecules or weakly physisorbed precursor mole-

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RESULTS AND DISCUSSION The FT-IR spectrum of the deposited Li2SiO3 film is shown in Figure 1. The bands present at 565 and 611 cm−1 are attributed to O−Si−O bending modes.29 The bands observed at 443 and 513 cm−1 are due to the deformation vibrations of the terminal Si−O− (Li+) type bonds and Li−O vibrations.30 These bands indicate that the lithium in the film is mainly present as Li+ ions. The bands at 735, 1021, and 1107 cm−1 correspond to Si−O stretching, characteristic of the silicon dioxide in lithium metasilicate.29 The broad band at 1400−1500 cm−1 is B



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films was around 2 rather than 0.5 as eqs 1 and 2 would imply, and hence it was proposed that some of the ligands are released in reactions with surface hydroxyl groups during the LiHMDS pulse, while only the remaining ligands are combusted during the ozone pulse.25 The decrease in the m0/m1 ratio with increasing temperature suggests that more fragments of the ligands remain adsorbed on the surface after the LiHMDS pulse. As more silicon containing fragments are present on the surface during the following ozone pulse, more silicon is also incorporated into the film. This explains the decrease of the Li/ Si ratio from 2.1 to 1.7 with the rise of the deposition temperature from 200 to 300 °C as observed by Hämäläinen et al.25 FT-IR results indicate that some intact methyl groups bonded to silicon remained in the film. These are the main source of carbon contamination. The amount of carbon in the film also decreased at higher deposition temperature (0.96 at.% at 200 °C, 0.32 at.% at 300 °C)25 due to a more complete combustion of the ligand fragments by ozone. For reference, we also examined adsorption of the protonated ligand HN[Si(CH3)3]2 that is expected to form if LiHMDS reacts with surface hydroxyl groups. After depositing lithium silicate film with 10 cycles of the LiHMDS−O3 process at 250 °C, HN[Si(CH3)3]2 seems to adsorb on the surface similarly to LiHMDS. This confirms previous studies that observed adsorption of HN[Si(CH3)3]2 through reaction with silanol groups.33,34 Figure 4 depicts possible adsorption and reaction steps of the LiHMDS precursor (compound 2 in Table 2) on a surface

Figure 2. QMS signals for significant byproducts of the LiHMDS−O3 process.

Figure 3. QCM results for two ALD cycles of the LiHMDS−O3 process at 250 °C.

cules from the surface. The ozone pulse causes a huge mass decrease, which is partially compensated during the following purge step. This apparent large mass drop is an artifact often observed with QCM on ozone based ALD processes.17 This artifact is due to the combination of pressure and temperature effects caused by the ozone pulses. A longer purging time than usual is required to compensate these effects. The mass change occurring during the LiHMDS pulse is noted m1 while the one after a complete cycle is noted m0. The m0/m1 ratio is different depending on the number of ligands or fragments of ligands remaining on the surface after the adsorption of LiHMDS. By observing the evolution of the m0/m1 ratio with temperature, variation in the film composition can be explained. An extreme case would be molecular or stoichiometric adsorption of LiHDMS followed by its complete combustion (eqs 1 and 2) that would result in an m0/m1 ratio of 0.27. LiN[Si(CH3)3 ]2 (g) → LiN[Si(CH3)3 ]2 (ads)

Figure 4. Possible reaction pathways for the adsorption of LiHMDS on a hydroxylated surface.

Table 2. Calculated Atomic Charges for the Optimized Compounds 1−5

(1)

LiN[Si(CH3)3 ]2 (ads) + 26.5O3(g) → (Li 2O)0.5

compound

formula

N

Li

H

Si

(SiO2 )2 + 6CO2 (g) + 9H 2O(g) + NO(g)

1

HN[Si(CH3)3]2

−1.493

-

0.398

2

LiN[Si(CH3)3]2

−1.823

0.844

-

3 4

LiNHSi(CH3)3 NH2Si(CH3)3

−1.601 −1.255

0.829 -

5

LiNH2

−1.452

0.808

0.366 0.373; 0.373 0.322; 0.322

0.52; 0.52 1.46; 1.46 1.42 0.49

+ 26.5O2 (g)

(2)

The m0/m1 ratio is measured to be 0.44 at 200 °C, 0.38 at 250 °C and 0.33 at 300 °C. These values indicate that the adsorption is not stoichiometric but rather involves a partial decomposition of the ligand, confirming the QMS results. Also, in the earlier study it was observed that the Li/Si ratio in the C

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the Si−N−Li bond angles. The optimized values are 126.86° and 95.81°. For the wider angle, the Si−N bond length is 1.707 Å, and for the other, the Si−N bond is 1.697 Å. The respective Wiberg bond indices are 0.758 and 0.822. The conformation of compound 5 is also interesting since it is planar in contrast to, e.g., NH3. This indicates that Li−NH2 is probably unstable and reacts rapidly on the surface. The presence of byproducts at the QMS signal m/z = 17 (corresponding to NH3) during the LiHDMS pulse validates this theory. The atomic charges calculated by NBO methods are shown in Table 2. While the atomic charges at Li and H have rather constant values, the atomic charge of N varies considerably from one compound to another: The lithium containing molecules exhibit larger negative charges on the N atom than the other compounds. The presence of the Li atom increases the polarity of both Li−N and N−Si bonds and thereby makes them more reactive. In the case of LiHMDS for instance, both N−Si and Li−N bonds are probably reacting with surface −OH groups: the differences of charges between the N and Si atoms (3.28) and N and Li atoms (2.67) in LiHMDS are sufficient to infer a high reactivity for both bonds. Even though LiHMDS and HMDS are molecules with almost similar structures, differing by only one atom (Li vs H), they exhibit quite important differences in their electronegativities: In LiHMDS, the electronegativity difference is 2.67 between the Li and N atoms, whereas in HMDS it is only 1.89 between the N and H atoms. Accordingly, in HMDS, the charge is −1.49 on the N atom, +0.52 on the Si atoms, and +0.40 on the H atom (Table 2). For the LiHMDS, the charge is +1.46 on the Si atoms, −1.82 on the N atom and +0.84 on the Li atom. The polarity of the N−Si bond is therefore higher than the polarity of the N−Li bond, enhancing its vulnerability to nucleophilic and electrophilic attacks from the surface −OH groups. This demonstrates the influence of the lithium atom in the precursor on the surface reactions allowing deposition at low temperature: the presence of the Li atom increases the electron density on the N atom and consequently the polarity of the surrounding N−Si bonds. The highest occupied molecular orbital (HOMO) of HMDS and LiHMDS displayed in Figure 6 illustrate this phenomenon: The presence of the

containing hydroxyl groups. In the gas phase, LiHMDS is mainly present as a dimer but a small percentage of monomers are observed too.33 Because of their lower coordination, monomers are expected to be more reactive than dimers and can dominate the reaction mechanism. Moreover, the interpretation of the QMS and QCM data leads to similar conclusions for both forms. Consequently, only the monomer is considered further on. Molecular or stoichiometric adsorption was ruled out based on the above argumentation. The LiHMDS precursor can react with the surface −OH groups by exchanging its lithium atom (path A) and forming a HN[Si(CH3)3]2 molecule (compound 1). HN[Si(CH3)3]2 can further react with surface OH groups, exchanging one of its trimethylsilyl group (path C). Alternatively, LiHMDS can also exchange one of its −Si(CH3)3 moieties with an −OH surface group (path B), thus generating a Li-NH−Si(CH 3 ) 3 intermediate (compound 3). The reaction continues further with the Li-NH-Si(CH3)3 intermediate exchanging either its lithium atom (path D) or its second −Si(CH3)3 moiety (path E) with the −OH surface groups. Once more, two byproducts are possibly formed: H2N−Si(CH3)3 (compound 4) after cleavage of the N−Li bond, and Li−NH2 (compound 5) after reaction with the −Si(CH3)3 moiety. Both can further react one last time with the remaining surface −OH groups to release NH3 (paths F and G). During the following ozone pulse, all the surface species left from LiHMDS are combusted producing lithium−silicon-oxide film and H2O, CO2, and NOx as detected with QMS. The methyl residues detected in the film indicate that the combustion reactions are not complete, however. Figure 5 shows the optimized structures of the main molecules (Table 2) involved in the reactions proposed in

Figure 5. The optimized structures of compounds 1−5 at the B3LYP/ 6-311G* level. (Color code: Si bluish gray, N blue, Li violet, C gray, H white.) Figure 6. Comparison between the HOMO of HMDS (left) and the HOMO of LiHMDS (right).

Figure 4. For each compound, only one conformation was found stable. The point groups for compounds 1−5 are C2, Cs, Cs, Cs, and C2v, respectively. For compounds 3 and 4, different conformations belonging to the point groups Cs were investigated too, but only the ones presenting no imaginary vibrations were considered. The position of Li in compounds 2 and 3 is noticeable as it is bent toward one of the methyl groups. In compound 2, the higher symmetry would have been C2v with the Li atom having equal distances to two methyl groups, but the calculations indicated that this structure was unstable. We carried out QTAIM analysis to observe whether a bond path between the Li atom and atoms of the adjacent methyl group was possible, but no bond paths indicating chemical bonding were found. There are marked differences in

lithium atom induces a shift of the π bond of Li−N in LiHMDS compared to the lone electron pair at the N atom in HMDS, leaving the Si−N bond more open for a nucleophilic attack in LiHMDS. HMDS is known to react preferentially with isolated and terminal hydrogen-bonded −OH groups on silica forming −O−Si(CH3)3 groups.34 The difference in polarity of the Si−N bonds of HMDS compared to LiHMDs points toward its lower reactivity, which can explain the high Li/Si ratio in the film. Similar behavior can be admitted for H 2 N−Si(CH 3 ) 3 (compound 4): The absence of a lithium atom reduces its reactivity toward the −OH surface groups, but H2N−Si(CH3)3 D



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When the same experiment was repeated on an Al2O3 surface, a mass increase was observed only during the first LiHMDS pulse but not anymore during the following pulses, thereby verifying that LiHMDS does not decompose at this temperature. The lack of “classic” self-limiting growth partially explains the strong dependence of the growth rate on temperature. The different reaction pathways form several intermediate species on the surface, which can either desorb or react further until a stable intermediate is reached. This leads to a balance on the surface between the stable intermediates and the dissociative adsorption of LiHMDS molecules. At a higher temperature the ligand of LiHMDS reacts more thoroughly, following the adsorption pathways depicted in Figure 4, thus leaving more Si in the film and creating less steric hindrance. This leaves more −OH surface groups available to react further with LiHMDS and other reaction intermediates (compounds 3 and 5), thus increasing the growth rate.

molecules can remain adsorbed on the surface and thus potentially block reactive sites. As the deposition temperature rises, the proportion of isolated −OH groups increases, and the adsorption of HMDS becomes easier, thus decreasing the Li/Si ratio. During the ozone pulses, both water and CO2 are formed upon the combustion of the remaining ligands of LiN[Si(CH3)3]2 and can further react with the film. Lithium oxide is highly reactive with H2O, especially in binary form, but apparently also in lithium silicates. Therefore a reservoir effect can occur where the film bulk absorbs water through the following reaction:35 (In this equation and all of the following, the underlined species denote the film bulk). Li 2O + H 2O → 2Li−OH

This reaction illustrates the hygroscopic behavior of lithium oxide. Consequently, reliable growth of lithium containing oxide films by ALD with water as the only oxygen precursor is extremely difficult.25,27,36 Carbon dioxide is also formed by combustion during the ozone pulse and can react further with Li−OH, forming lithium carbonate and water.

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CONCLUSIONS The mechanism of the LiHMDS−O3 ALD process for lithium silicate was studied in situ and its characteristics explained: The QCM and QMS results indicate that the ligand of LiHMDS decomposes upon arriving on the surface by successively reacting with −OH groups. This is rendered possible by the presence of the lithium atom in the LiHMDS molecule, which enhances the reactivity of not only the Li−N but also the N−Si bond as compared with HMDS, a protonated form of the ligand. Along the adsorption pathways, nonreactive intermediates are formed that can limit the access of LiHMDS and other intermediates to reactive sites. Lithium is incorporated into the film as Li+ ions and silicon forms −Si(CH3)3 surface groups, which are then combusted during the following ozone pulse leading to an incorporation of SiO2 into the film. When the deposition temperature rises, the adsorption reactions become more efficient, and more −Si(CH3)3 groups remain on the surface after the LiHDMS pulse decreasing the Li/Si ratio in the film. Moreover, the higher the deposition temperature, the faster nonreactive molecules desorbed and made more surface −OH groups available for further reactions, which increases the growth rate.

2Li−OH + CO2 → Li 2CO3 + H 2O

Besides during the film growth, such a reaction can also occur after the deposition when the film is exposed to air. This explains the Li2CO3 band observed with FT-IR and the necessity to use good capping to prevent the rapid degradation of the lithium containing layer.37,38 When the reaction between the film and water occurs extensively, the growth is not anymore limited only by the reactive groups on the surface because the hydroxyl groups in the film bulk can also participate in the reactions with LiHMDS/LiN[Si(CH3)3]2. Figure 7 illustrates this absence of

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AUTHOR INFORMATION

Corresponding Author

*E-mail: tomczak.yoann@helsinki.fi. Tel: +358449700585. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research leading to these results have received funding from the European Community’s Seventh Framework Programme (FP7/2007-2013) under grant agreement number ENHANCE-238409, and from the Finnish Centre of Excellence in Atomic Layer Deposition, funded by the Academy of Finland.

Figure 7. QCM data corresponding to 10 cycles of the LiHMDS−O3 process followed by 10 LiHMDS pulses.

surface limited growth: When several LiHMDS pulses are applied successively on a lithium silicate film deposited with 10 cycles of the LiHMDS−O3 process, repetitive mass increases can be observed with QCM. This is explained by the following reaction:

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Li−OH + LiN[Si(CH3)3 ]2 → Li 2O + HN[Si(CH3)3 ]2

REFERENCES

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Surface and bulk hydroxyl groups react with LiHMDS releasing HN[Si(CH3)3]2 as byproducts. The mass gain slightly diminishes after each LiHMDS pulse (Figure 7), indicating a slow depletion of the hydroxyl groups also from the film bulk. E



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