Molecular Layer Deposition of Aluminum Alkoxide Polymer Films

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Molecular Layer Deposition of Aluminum Alkoxide Polymer Films Using Trimethylaluminum and Glycidol Younghee Lee,† Byunghoon Yoon,† Andrew S. Cavanagh,‡,§ and Steven M. George*,†,§,^ †

Department of Chemistry and Biochemistry, ‡Department of Physics, §DARPA Center for Integrated Micro/Nano-Electromechanical Transducers (iMINT), and ^Department of Chemical and Biochemical Engineering, University of Colorado at Boulder, Boulder, Colorado 80309, United States ABSTRACT: Molecular layer deposition (MLD) of aluminum alkoxide polymer films was examined using trimethlyaluminum (TMA) and glycidol (GLY) as the reactants. Glycidol is a high vapor pressure heterobifunctional reactant with both hydroxyl and epoxy chemical functionalites. These two different functionalities help avoid “double reactions” that are common with homobifuctional reactants. A variety of techniques, including in situ Fourier transform infrared (FTIR) spectroscopy and quartz crystal microbalance (QCM) measurements, were employed to study the film growth. FTIR measurements at 100 and 125 C observed the selective reaction of the GLY hydroxyl group with the AlCH3 surface species during GLY exposure. Epoxy ring-opening and methyl transfer from TMA to the surface epoxy species were then monitored during TMA exposure. This epoxy ring-opening reaction is dependent on strong Lewis acid
base interactions between aluminum and oxygen. The QCM experiments observed linear growth with self-limiting surface reactions at 100
175 C under certain growth conditions. With a sufficient purge time of 20 s after TMA and GLY exposures at 125 C, the mass gain per cycle (MGPC) was 19.8 ng/cm2-cycle. The individual mass gains after the TMA and GLY exposures were also consistent with a TMA/GLY stoichiometry of 4:3 in the MLD film. This TMA/GLY stoichiometry suggests the presence of Al2O2 dimeric core species. The MLD films resulting from these TMA and GLY exposures also evolved with annealing temperature to form thinner conformal porous films with increased density. Non-self-limiting growth was a problem at shorter purge times and lower temperatures. With shorter purge times of 10 s at 125 C, the MPGC increased dramatically to 134 ng/cm2-cycle. The individual mass gains after the TMA and GLY exposures in the CVD regime were consistent with a TMA/GLY stoichiometry of 1:1. The MGPC decreased progressively versus purge time. This behavior was attributed to the removal of reactants that could lead to CVD and the instability of the surface species after the reactant exposures. These results reveal that the TMA and GLY reaction displays much complexity and must be carefully controlled to be a useful MLD process.

I. INTRODUCTION Molecular layer deposition (MLD) is a growth process based on sequential, self-limiting surface reactions that deposits organic or hybrid organic
inorganic films.1 MLD is similar to atomic layer deposition (ALD) which has been shown to deposit extremely conformal inorganic films with Angstrom-level control of the film thickness.2 MLD techniques have been developed for depositing organic polymer films such as polyamides,3
5 polyimides,6,7 and polyureas.8,9 MLD can also be employed to deposit hybrid organic
inorganic films by combining inorganic ALD precursors with organic MLD precursors.1,10
17 One of the first hybrid organic
inorganic MLD systems was alucone MLD based on trimethylaluminum (TMA) and ethylene glycol (EG) as the reactants.11 Zincone MLD has also been demonstrated using diethylzinc (DEZ) and EG as the reactants.16,17 These MLD reactions displayed some difficulty using a homobifunctional precursor such as EG. Both hydroxyl groups on EG can react twice on the surface and yield a “double reaction”.3,11 These double reactions restrict the number of active sites on the surface and impede the propagation of a polymer chain. r 2011 American Chemical Society

One solution to prevent “double reactions” is to employ heterobifunctional precursors and ring-opening reactions.1 Following this strategy, a three-step ABC reaction sequence with a heterobifunctional precursor and a ring-opening reaction was demonstrated using TMA, ethanolamine, and maleic anhydride.18,19 This ABC reaction sequence displayed exceptionally large growth rates that argued that “double reactions” were not a problem. However, TMA diffusion was observed into the growing film during the TMA exposures.19 This extra TMA inside the growing film led to an additional chemical vapor deposition (CVD) reaction that was identified as the main reason for the large growth rates.19 Glycidol (GLY) is a heterobifunctional precursor that includes both a hydroxyl group and an epoxy ring that may avoid “double reactions”. GLY also has an excellent vapor pressure of ∼1 Torr at 20 C. This vapor pressure is much higher than the EG vapor pressure of ∼0.1 Torr at 20 C. Because of the excellent reactivity Received: June 24, 2011 Revised: October 6, 2011 Published: October 26, 2011 15155

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of TMA and EG, GLY should react efficiently with TMA and also avoid possible “double reactions”. Our group and another group reported the reaction of TMA and GLY at conferences in 2010 and 2011.20,21 A recent publication on the reaction of TMA and GLY has also appeared in the literature.22,23 In this paper, the reaction of TMA and GLY is characterized over a wide range of temperatures, reactant exposures, and purge times. The results reveal that the TMA and GLY MLD reaction is efficient and the observed film growth is linear versus the number of MLD cycles under certain reaction conditions. However, there are complications with competing chemical vapor deposition (CVD) processes at lower temperatures and shorter purge times. There also may be instability of surface species after reactant exposures that lead to mass losses versus time. Consequently, the film growth per MLD cycle is not constant versus changing reaction parameters and the TMA and GLY MLD reaction requires more attention to reaction conditions than other MLD processes.

II. EXPERIMENTAL SECTION A. Viscous Flow Reactors for in Situ FTIR Spectroscopy and QCM Measurements. The viscous flow reactor equipped with an in situ FTIR spectrometer has been described previously.4,11,17 The reactants were pumped by a liquid nitrogen trap and a mechanical pump (Pascal 2015SD, Alcatel). The mechanical pump maintained a pressure of ∼1 Torr with a continuous nitrogen gas (Ultrahigh purity, Airgas) flow of 150 sccm into the reactor was supplied by mass flow controllers (Type 1179A, MKS). Pressure was measured using a bakeable capacitance manometer (Baratron 121A, MKS). Nitrogen gas was used as a carrier gas and a purge gas in the viscous flow reactor. The FTIR experiments were performed using a Nicolet Nexus 870 FTIR spectrometer equipped with a liquid-nitrogen-cooled mercury
cadmium
telluride infrared detector. To obtain sufficient surface area for transmission FTIR experiments, ZrO2 nanoparticles (99%, Alfa Aesar) with a diameter of 30
50 nm and a surface area of 20
30 m2 g
1 were pressed into a stainless steel grid (Tech-Etch).24,25 Transparent KBr windows (polished, International Crystal Laboratories) were used to transmit the infrared beam through the reactor. The TMA (97%, Sigma-Aldrich) was held at room temperature. For the TMA reaction, each TMA exposure consisted of ten 1.0 s doses at a partial pressure of 120 mTorr. The GLY (Glycidol, 96%, Sigma-Aldrich) precursor was degassed by freeze
pump
thaw cycles and was maintained at 40
45 C. Each GLY exposure consisted of fifteen 1.0 s doses at a partial pressure of 30 mTorr. These multiple doses were required to reach completion of the surface chemistry on the high surface area ZrO2 substrate. Various purge times were used after each TMA dose and GLY dose. One set of TMA and GLY exposures defined one MLD cycle. The viscous flow reactor equipped with an in situ QCM has also been described previously.26 In situ QCM measurements were conducted using a film deposition monitor (Maxtek TM-400, Inficon). The QCM sensor with quartz crystal (polished and gold coated AT-cut, 6 MHz, Colorado Crystal Corp.) was mounted and sealed with high-temperature epoxy (Epo-Tek H21D, Epoxy technology) in the bakeable single sensor head (BSH-150, Inficon). A PID temperature controller (2604, Eurotherm) maintained a constant temperature in the reactor to within (0.04 C. The QCM studies of the MLD reaction using TMA and GLY were performed at 100, 125, 150, and 175 C. Before each MLD experiment, at least 200 cycles of Al2O3 ALD using TMA and H2O (deionized water, Chromasolv for HPLC, Sigma-Aldrich) were deposited on the crystal to prepare a reproducible surface. The TMA exposure was a 1.0 s dose at a partial pressure of 25 mTorr. The GLY exposure was a 1.5 s dose at a partial pressure of 35 mTorr. Single doses were sufficient

Figure 1. Absolute FTIR spectra during MLD film growth using the TMA and GLY reaction after 1, 3, 5, 7, 9, and 15 cycles at 125 C. After the first cycle, the spectra are recorded following the GLY reaction. for the completion of the surface chemistry on the low-surface-area QCM sensor.

B. X-ray Reflectivity, Ellipsometry, and X-ray Photoelectron Spectroscopy Measurements. The ex situ X-ray reflectivity (XRR) scans were obtained using a high-resolution X-ray diffractometer (Bede D1, Jordan Valley Semiconductors) using Cu Kα (λ = 1.540 Å) radiation. All XRR scans were fit using Bede REFS software (Jordan Valley Semiconductors) to analyze film thickness, surface roughness, and film density. Boron-doped Si (100) wafers (p-type, Silicon Valley Microelectronics) were used for the substrates. The film thicknesses were also determined using ex situ reflective spectroscopic ellipsometry investigations. These measurements were performed using a J. A. Woollam M-2000 spectroscopic ellipsometer employing a spectral range from 240 to 1700 nm with an incidence angle of 75. The film thicknesses were obtained using a Kramers
Kronig consistent B-spline formulation for the dielectric function representation.27 The refractive index at 587 nm was 1.51. X-ray photoelectron spectroscopy (XPS) was performed using a PHI 5600 X-ray photoelectron spectrometer. Monochromatic Al Kα X-rays (1486.6 eV) were used for the XPS analysis. A depth profile was obtained using Ar ion sputtering. Data was collected with Auger Scan (RBD Enterprises, Inc., Bend, OR). The XPS data was analyzed in CASA XPS (Casa Software Ltd., UK).

III. RESULTS AND DISCUSSION A. Fourier Transform Infrared Spectroscopy Studies. The MLD growth based on TMA and GLY reactants was investigated using in situ transmission FTIR analysis at 100 and 125 C. The in situ FTIR spectra were recorded after each reactant exposure. Figure 1 shows the absolute FTIR spectra versus cycle number on high surface area ZrO2 nanoparticles at 125 C. Nearly identical results were obtained at 100 C. All the FTIR spectra were referenced to the spectrum for the KBr windows. The FTIR spectra are displaced for clarity in presentation. 15156

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Langmuir Figure 1a shows the FTIR spectra of the hydroxylated ZrO2 nanoparticles. A broad infrared absorbance at 2470
3810 cm
1 displays O
H stretching vibrations on the ZrO2 surface. Strong bulk absorption of ZrO2 is also observed at 125 C. In contrast, no mass loss was observed at 100 C. F. Annealing of MLD Films. Previous studies have revealed that the organic constituent of hybrid organic
inorganic MLD films can be removed to yield porous metal oxide films.48,49 Alucone MLD films grown using TMA and EG were converted 15161

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Figure 14. (a) Thickness and (b) electron density from XRR scans of films in Figure 13 versus thermal annealing. The thicknesses were also obtained from spectroscopic ellipsometry (SE) and shown for comparison.

to porous alumina by thermal annealing at 400 C.48,50 After annealing, these porous metal oxide films have a distribution of micropores and mesopores.48 These porous alucone films have been used recently for gas separation membranes.49 The thermal stability of the MLD films grown using TMA and GLY was studied using XRR. The loss of organic constituent was studied by monitoring the thickness and density of the MLD film versus annealing temperature. XPS depth-profiling analysis confirmed that the annealed MLD films at 350 and 500 C were Al2O3 and did not contain carbon. The annealing was performed by placing the samples in a furnace set to a specific temperature for 24 h. Figure 13 displays XRR scans for MLD films grown using 400 cycles of TMA and GLY at 125 C on a Si(100) substrate. The pronounced interference fringes in Figure 13 over many orders of magnitude of intensity reduction are consistent with very smooth conformal films. In addition, the changing period of the interference fringes indicates that the MLD films become progressively thinner versus annealing temperature. A shift in the critical angle to larger angles with annealing temperature also indicates that the films are densifying versus annealing. The thickness and electron density of the MLD films versus annealing temperature are shown in Figure 14. The electron density is reported during annealing because determining the mass density from the critical angle requires knowledge of the film composition.51,52 The mass density, Fm, can be related to the electron density by Fm = (NeA)/(NAZ) where A is the average molar mass, Z is the average atomic number, and NA is Avogadro’s number.51,52 The mass density at 25 C before the annealing can be assigned by assuming a film composition of Al(CH3)1.5(C4H8O2)0.75 or Al2C9H21O3 based on the reaction stoichiometry of 4:3. The mass density can be determined after the annealing at 500 C by knowing that the film is Al2O3. Although the MLD films shrink and increase in electron density versus annealing temperature, the resulting metal oxide films are porous. The mass density of 2.2 g/cm3 after annealing at 500 C is significantly less than the density of Al2O3 ALD films of

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3.0 g/cm3 and the density of α-Al2O3 crystalline films of 3.97 g/ cm3. On the basis of the densities of Al2O3 ALD and α-Al2O3 crystalline films, the MLD films annealed to 500 C would have a porosity between 27% and 45%. G. Comparison with Other Studies. There are some similarities and differences between this study and a recently published study.22,23 Both investigations observed film growth using TMA and GLY and noticed considerable temperature dependence for the film growth rate. Both investigations have also noted the importance of reactant absorption to explain the temperature dependence of film growth and explored thermal annealing. The key differences between the studies are the interpretation of the reaction mechanism and the magnitude of the growth rates. The recently published study observed O
H stretching vibrations in the FTIR spectrum following GLY exposures during the TMA and GLY reaction.22 On the basis of this observation, they concluded that some GLY reactants react with AlCH3*surface species via the epoxy group. In contrast, this study observed no O
H stretching vibrations in the FTIR spectra in Figure 2 either as a gain in absorbance during GLY exposure or as a loss in absorbance during TMA exposure. The O
H stretching vibrations observed in the recently published study are believed to be an artifact from H2O in the GLY resulting from incomplete freeze
pump
thawing. In addition, the observation of O
H stretching vibrations may also be the result of the underlying oxide surface and not reaching the steady-state growth conditions. The recently published study also observed much larger growth rates than the growth rates observed in the current study.22 Even after accounting for the unpolished QCM crystal used in the other study, the growth rates obtained by their QCM measurements are much larger and would be considered to be in the CVD regime based on the results of the current study. The CVD regime occurs at lower temperatures and after short purge times as a result of excess reactant remaining in or on the MLD film during the next reactant exposure. Longer purge times and more effective pumping may be needed to avoid the CVD regime for the experiments performed primarily at 120 C in the other study. 22

IV. CONCLUSIONS The MLD reaction using TMA and GLY was examined to understand the film growth between TMA and the heterofunctional GLY precursor that contains both a hydroxyl and an epoxy group. The heterofunctional GLY precursor should prevent “double reactions” that may limit MLD film growth. The TMA and GLY reaction was studied versus many reaction conditions including temperature, reactant exposure, and purge time. The results revealed that this MLD reaction is very sensitive to reaction conditions. Much larger growth rates are observed at lower temperatures and at shorter purge times when excess reactant is still absorbed and leads to a CVD reaction with the subsequent reactant. Under conditions of a sufficient reactant purge time of 20 s at 125 C, growth rates of 19.8 ng/cm2-cycle or 1.3 Å/cycle were observed using QCM and XRR measurements. Shortening the purge time to 10 s led to much larger growth rates of 134 ng/cm2cycle. A similar dramatic increase in growth rate was observed at shorter purge times for growth at 100 C. In addition to the larger growth rate, the ratio of mass gains observed for the individual TMA and GLY reactions also changed between the lower and higher growth regimes. The ΔMTMA/(ΔMTMA+ΔMGLY) ratio 15162

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Langmuir was 0.63 for lower growth rates and decreased to 0.55 for the higher growth rates that are in the CVD regime. No O
H stretching vibrations were observed in the FTIR spectra after GLY exposures during the TMA and GLY reaction. The absence of O
H stretching vibrations indicates that GLY reacted with AlCH3 species through its hydroxyl group. The FTIR spectra were consistent with TMA opening the epoxy ring and also transferring a methyl group during TMA exposures. Assuming this reaction mechanism, the TMA/GLY stoichiometries can be determined based on the ΔMTMA/ (ΔMTMA+ΔMGLY) ratio. The ratio of 0.63 observed during lower growth rates was consistent with a 4:3 TMA/GLY stoichiometry. TMA/GLY stoichiometries larger than 1:1 may be explained by Al2O2 dimeric cores in the MLD film. The ratio of 0.55 observed during higher growth rates was consistent with a 1:1 TMA/GLY stoichiometry. The reaction mechanism for TMA and GLY is dependent upon the chemical properties of TMA. Experiments with DEZ and GLY revealed negligible growth indicating that the Lewis acidity of TMA may be critical for TMA adsorption and epoxy ring-opening. The MLD films showed mass loss after growth that may be consistent with the instability of the surface species. In addition, the MLD films evolved with annealing temperature to form thinner conformal porous films with increased density. MLD film growth with TMA and GLY displays an interesting reaction mechanism and much complexity. Compared with other MLD processes, the TMA and GLY reaction requires more attention to reaction conditions to be used as a controllable and reliable MLD process.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This research was funded by the National Science Foundation (CHE-1012116) and the Laboratory Directed Research and Development (LDRD) Program at the National Renewable Energy Laboratory. NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC. Equipment utilized in this work was also provided by the Air Force Office of Scientific Research. The authors also thank Ms. Noemi Leick-Marius from the Dept. of Applied Physics, Eindhoven University of Technology for discussions regarding spectroscopic ellipsometry and the thickness of the aluminum alkoxide films. ’ REFERENCES (1) George, S. M.; Yoon, B.; Dameron, A. A. Acc. Chem. Res. 2009, 42, 498. (2) George, S. M. Chem. Rev. 2010, 110, 111. (3) Adamczyk, N. M.; Dameron, A. A.; George, S. M. Langmuir 2008, 24, 2081. (4) Du, Y.; George, S. M. J. Phys. Chem. C 2007, 111, 8509. (5) Shao, H. I.; Umemoto, S.; Kikutani, T.; Okui, N. Polymer 1997, 38, 459. (6) Putkonen, M.; Harjuoja, J.; Sajavaara, T.; Niinisto, L. J. Mater. Chem. 2007, 17, 664. (7) Yoshimura, T.; Tatsuura, S.; Sotoyama, W. Appl. Phys. Let.t 1991, 59, 482.

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