Cross-Linked Ultrathin Polyurea Films via Molecular Layer Deposition

Jul 11, 2013 - Ultrathin cross-linked polymer thin films are highly desirable materials because of their important roles in many applications. However...
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Cross-Linked Ultrathin Polyurea Films via Molecular Layer Deposition Han Zhou,† Michael F. Toney,‡ and Stacey F. Bent*,§ †

Department of Chemistry, Stanford University, Stanford, California 94305, United States Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States § Department of Chemical Engineering, Stanford University, Stanford, California 94305, United States ‡

S Supporting Information *

ABSTRACT: Ultrathin cross-linked polymer thin films are highly desirable materials because of their important roles in many applications. However, they are difficult and challenging to fabricate. Here we report a one-step process for depositing cross-linked polyurea thin films using a vapor-phase molecular layer deposition (MLD) technique. 1,4-Diisocyanatobutane and a series of different multiamines, including diethylenetriamine, triethylenetetramine, and tris(2-aminoethyl)amine, were used to grow polyurea MLD films via urea-coupling reactions. The deposited cross-linked polyurea films exhibit characteristic MLD film growth behaviors, such as constant growth rates, infrared absorption by expected urea modes, and stoichiometric chemical compositions. More importantly, the cross-linking is shown to be capable of improving the film properties. Based on cross-linking, the thin film density can be increased by approximately 50%. In addition, the film decomposition temperature is increased by about 30 °C, suggesting an enhanced thermal stability of the cross-linked ultrathin polyurea films.

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exquisite control over film thickness, composition, and conformality at the molecular level.7,12 Using this technique, polyamide,13−16 polyimide,17−19 polyurea,20−23 polythiourea,24 and polyurethane25 films have been deposited. MLD offers a number of advantages over other deposition methods.26 First, angstrom-level control over film thickness can be achieved due to the self-saturating nature of the coupling reactions. Second, film composition can be finely tuned by incorporating desired functionalities into the backbone of precursor molecules. Third, MLD can provide highly conformal coatings even on substrates with high-aspect-ratio patterns.24 These advantages of MLD can be desirable in various applications, especially those in which a high level of control over the surface or interfacial coating is needed.26 Therefore, MLD is beginning to be explored for application in a variety of technological areas such as microelectronics,23,27,28 catalysis,29,30 and material modification.31,32 Although a variety of ultrathin organic thin films have been successfully deposited using MLD, the vast majority of these are comprised of linear polymeric chains, resulting from the use of bifunctional monomers. Sung et al. fabricated crosslinked inorganic−organic hybrid MLD thin films by crosslinking the acetylene groups embedded in the backbone of the hexadiyne-diol precursor.33,34 However, UV radiation was needed to trigger the cross-linking reaction, leading to possible damage to the deposited films. In addition, the film contained zinc oxide and titanium oxide inorganic groups. Hence, it is

INTRODUCTION Cross-linked organic thin films are desirable materials due to their improved properties compared to the non-cross-linked films, including higher film density, better mechanical strength,1 and higher thermal stability.2 With these advantages, crosslinked organic thin films have been used in many applications where non-cross-linked films are not suitable, such as organic thin film transistors,3 humidity sensors,4 and purification membranes.5,6 However, the deposition of cross-linked organic thin films is more challenging than that of non-cross-linked ones. Cross-linked polymers are usually less soluble in solvents, leading to difficulty in direct spin-coating, a process which is widely used for the deposition of non-cross-linked polymeric thin films. To address this challenge, a two-step spin-coating process was developed, in which a non-cross-linked polymeric film is deposited first, followed by a post-treatment to introduce cross-links.7 The drawback of this method is that it can require harsh conditions, such as elevated temperature or photoradiation, 8 which can damage heat- or photosensitive functionalities. Recently, several solution-based layer-by-layer depositions of cross-linked polyamide6,9,10 and polyurea11 thin films have been reported. This approach consists of a series of sequential dipping or spin-coating steps, with each solution applied containing only one multifunctional monomer, and it produces highly cross-linked polymeric thin films via spontaneous reaction between different monomers. Although mild conditions are used in this approach, the whole process requires many dipping and rinsing steps, which can be tedious and can consume large volumes of solvent. Vapor-based molecular layer deposition (MLD) is an emerging technique for growing organic thin films, providing © 2013 American Chemical Society

Received: May 16, 2013 Revised: June 28, 2013 Published: July 11, 2013 5638

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Figure 1. (a) Chemical structures of precursors used in this work. (b) Schematic of MLD processes for (i) non-cross-linked DICB/ED and (ii) cross-linked DICB/DETA films, showing both ideal and nonideal processes. purchased from Sigma-Aldrich and used as received. MLD films were grown on silicon (100) wafers with a 4 nm thermal oxide. Prior to MLD, silicon wafers were cleaned with piranha solution, followed by a surface modification using APTES vapor, with the process described in a previous study.21 After the APTES treatment, the reactor was cooled down to room temperature for MLD cycling. DICB, DETA, and ED were also kept at room temperature while TETA and TAEA were heated with a 40 °C oil bath to increase their volatility. Each complete binary MLD cycle involves a DICB dose, followed by purging with nitrogen, a DETA (or TETA, TAEA, or ED) dose, followed by another purging with nitrogen. The MLD dose and purging times (in minutes) for DICB/ED, DICB/DETA, DICB/TETA, and DICB/ TAEA films in the sequence of DICB dose/N2 purging/amine precursor dose/N2 purging are 7.5/4/3/5, 12/6/12/6, 14.5/4/14.5/4, and 14.5/4/14.5/4, respectively. The dose lengths were chosen to ensure enough reaction time to achieve saturation for each precursor. Interestingly, the required DICB dose time increased with increasing steric hindrance of the amine counter reactant, from ED to DETA to TETA and TAEA, with the bulkier precursors likely requiring longer exposure times to drive the reaction to completion. Following MLD, samples were taken out of the reactor for ex situ analysis and characterization. A Gaertner Scientific Corp. L116C He−Ne laser ellipsometer with 632.8 nm light and a J.A. Woollam alpha-SE laser spectroscopic ellipsometer were used for ellipsometry measurements. The sample thickness was measured in at least three different locations on each sample so as to test film uniformity across it. For films measured using the Gaertner Scientific Corp. L116C He−Ne laser ellipsometer, a refractive index of 1.46 was used for both the SiO2 and organic film because the refractive indices of both materials are very close.35−37 The SiO2 thickness of piranha-cleaned silicon samples was measured and used as a baseline oxide thickness. The thickness of the organic film deposited above the SiO2 layer was obtained by subtracting the baseline SiO2 thickness from the total film thickness values. For the J.A. Woollam alpha-SE laser spectroscopic ellipsometer, a Cauchy

desirable to develop MLD processes for depositing ultrathin cross-linked organic films using mild conditions. In this study, we report a series of highly cross-linked polyurea thin films grown by MLD using a combination of bifunctional and multifunctional precursors. To our knowledge, direct deposition of a cross-linked polymer by MLD has not been previously reported. Specifically, 1,4-diisocyanatobutane (DICB) was used as the bifunctional monomer, while diethylenetriamine (DETA), triethylenetetramine (TETA), and tris(2-aminoethyl)amine (TAEA) were used as the multifunctional counter monomers. All the MLD films showed constant growth rates. Fourier transform infrared (FTIR) spectroscopy confirmed the MLD films were deposited via the urea-coupling reaction between isocyanate and amine groups. X-ray photoelectron spectroscopy (XPS) demonstrated stoichiometric compositions of the MLD films. Moreover, compared with the non-cross-linked polyurea films deposited with bifunctional DICB and ethlyenediamine (ED), the crosslinked MLD films exhibit higher film density and better thermal stability, as shown by X-ray reflectivity (XRR) measurements and annealing experiments, respectively.

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EXPERIMENTAL SECTION

MLD films were deposited in a flow reactor pumped by a Leybold Trivac rotary vane pump with a base pressure below 1 mTorr. The reactor was heated by a heating tape which was controlled by a variable transformer. The precursors and nitrogen purge gas were introduced into the reactor using Swagelok ALD valves controlled by a LabVIEW program. 3-Aminopropyltriethoxysilane (APTES), 1,4-diisocyanatobutane (DICB), diethylenetriamine (DETA), triethylenetetramine (TETA), tris(2-aminoethyl)amine (TAEA), and ethylenediamine (ED) were all 5639

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model was used to calculate the film thickness. FTIR spectra were collected with a Thermo Nicolet 6700 FTIR spectrometer using a MCT-A detector in transmission mode. Each spectrum was taken with 200 scans at a resolution of 4 cm−1. Piranha-cleaned silicon samples were used as a background reference for FTIR measurements. XPS measurements were performed on a Physical Electronics, Inc. 5000 Versaprobe spectrometer using an excitation source of Al Kα radiation (1486.6 eV). Atomic compositions were calculated by determining peak areas, and peaks were fit using Gaussian−Lorentzian profiles with a Shirley background. Film density evaluation was obtained using XRR measurements, which were performed on the Beamline 2-1 at the Stanford Synchrotron Radiation Lightsource (SSRL). The X-ray wavelength was 1.5498 Å, and all data are reported as a function of the scattering vector Q = (4π/λ) sin θ, where λ is the X-ray wavelength and θ is half the scattering angle. Angular collimation for the reflected X-rays was set by two 1 mm slits spaced by about 400 mm. Simulation of the reflectivity data profiles was performed using the StochFit38 software package, with a multibox model and minimum constraints to obtain a good fit.

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RESULTS AND DISCUSSION The MLD process for depositing cross-linked polyurea thin films consists of alternating doses of DICB and amine precursors, starting with an amine-terminated surface after the initial APTES functionalization. Figure 1 depicts the chemical structures of the monomers employed in this work and schematics of the MLD processes for the non-cross-linked and cross-linked polyurea thin films, with both ideal and nonideal growth behaviors shown. For the non-cross-linked DICB/ED film, during each half cycle in an idealized system, the bifunctional precursors use one of the functional groups to form a urea linkage with a surface reactive site, leaving the other functional group available for the next half cycle. As a result, it is expected that the MLD film comprises individual polymer chains without covalent interconnections between adjacent chains. In contrast, for the cross-linked polyurea films, as depicted in Figure 1b with a triamine precursor, e.g. DETA, if the multiamine precursor uses one amine group to attach itself to the surface, two amine groups will be left for subsequent film growth. Consequently, the concentration of surface reaction sites increases, which in turn can facilitate the formation of a dendrimer-like polymer network. Moreover, the multifunctional amine precursors may also serve as bridging elements between adjacent polymer chains; for instance, two amine groups can react with two neighboring surface isocyanate groups during a half cycle, with the third amine group retained for further film growth. As a result, a polyurea film with crosslinked polymer chains can be deposited. To demonstrate the layer-by-layer growth behavior of MLD, film thicknesses were measured after different numbers of MLD cycles for each reaction system, as shown in Figure 2a. It can be seen that linear growth behavior was observed for all MLD films, confirming the characteristic MLD growth. The growth rates for DICB/ED, DICB/DETA, DICB/TETA, and DICB/ TAEA MLD films are 6.3, 6.7, 3.2, and 3.1 Å/cycle, respectively. These growth rates are smaller than the chain lengths of one binary DICB/amine molecular unit, which are estimated to be approximately 13.5, 17.2, 20.9, and 17.2 Å, respectively, based on the constituent bond lengths and angles in the molecular units. The deviation of the growth rates from the unit chain lengths can be attributed to several factors. First, the polymer chains of the MLD films are not normal to the substrate surface. In fact, a tilted configuration of MLD polymer chains has been predicted by theoretical calculations21and recently observed in experiments.39 Second, it is possible that a fraction

Figure 2. (a) Film thicknesses as a function of number of MLD cycles for DICB/ED, DICB/DETA, DICB/TETA, and DICB/TAEA polyurea MLD films. (b) FTIR spectra of as-deposited (i) DICB/ ED, (ii) DICB/DETA, (iii) DICB/TETA, and (iv) DICB/TAEA polyurea MLD films.

of the incident precursors may have all of its functional groups, either amine or isocyanate, reacted with the surface, forming a “capped” region on the reactive film surface, as illustrated in Figure 1b. This capping effect terminates the surface with a nonreactive, alkyl chain backbone, which in turn reduces the amount of surface reactive sites and leads to a lower nucleation density and slower growth rate.15 Third, it is noticeable that the DICB/TETA and DICB/TAEA films have slower growth rates than the DICB/ED and DICB/DETA films. This is likely caused by steric hindrance due to the bulky hyper-branched TETA and TAEA precursors, which may affect the accessibility of surrounding reactive sites and cause submonolayer coverage. Nonetheless, although the growth rates of the MLD polyurea films are smaller than the unit lengths, continuous film deposition is observed since the film thickness continues increasing with increasing number of MLD cycles. Linear growth alone does not necessarily confirm that the MLD films were built upon urea-coupling reactions between the isocyanate and amine groups. To confirm the coupling reaction, we carried out additional experiments. FTIR spectroscopy measurements were performed to examine the bonding within the MLD films, as shown in Figure 2b. Notably, the spectra of the four MLD films look identical, which agrees with expectation since the chemical bonding is essentially the same for all the MLD films regardless of the degree of cross-linking. Characteristic IR peaks of the urea linkage, including ν(N−H) at 3334 cm−1, ν(CO) at 1635 cm−1 (amide I mode), and δ(N−H) at 1559 cm−1 (amide II mode),40 were present in all of the MLD films. It is also seen that compared to the ν(CO) stretch mode observed at 1651 cm−1 in aromatic polyurea films deposited with p-phenylene diisocyanate (PDIC) and ethylenediamine (ED),21 i.e., the 5640

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aromatic counterpart of MLD films formed with DICB, the ν(CO) peaks of the aliphatic polyurea films grown in this work were red-shifted by 16 cm−1. This red-shift suggests that the aliphatic polyurea MLD films may have stronger hydrogen bonding between the carbonyl oxygen and amide N−H moieties.41 The aliphatic polymer chains are likely more flexible than the aromatic ones, due to the absence of rigid aromatic rings in the backbone. The increased chain flexibility can in turn promote the formation of hydrogen bonding between adjacent urea moieties. Besides the confirmation of urea linkages by FTIR spectroscopy, chemical composition analysis of the cross-linked polyurea MLD films was performed using XPS. For all of the MLD films studied, only C, N, and O signals were observed in the XPS spectra, agreeing with the expected constituent elements of the MLD films. Table 1 shows the chemical

(Supporting Information). From Figure 3 it can be seen that the reflectivity curve shows many oscillations, indicating a smooth film of good quality. Figure 3 also shows a good fit obtained using the StochFit program. Data fitting gives a film thickness of 9.0 ± 0.1 nm, which matches very well with the ellipsometric value of 8.6 nm, suggesting a good agreement between the two thickness measurement techniques. The corresponding electron density profile, together with a box model fit, is shown in the inset of Figure 3. The fitting was repeated for all of the MLD films. Table 2 lists the normalized Table 2. Polyurea MLD Film Thicknesses and Normalized Electron Densities Obtained from XRR Data Fitting and Film Thicknesses Measured by Ellipsometry MLD film

Table 1. Chemical Compositions of Cross-Linked MLD Films Obtained by Both Calculation (Calc) and XPS Measurement (Expt) MLD film DICB/DETA DICB/TETA DICB/TAEA

calc expt calc expt calc expt

C (%)

N (%)

O (%)

59.1 59.3 60.0 62.5 60.0 58.7

27.3 27.7 26.7 26.5 28.0 28.3

13.6 13.0 13.3 11.0 12.0 13.0

DICB/ED DICB/DETA DICB/TETA DICB/TAEA

normalized electron density 0.37 0.56 0.49 0.56

± ± ± ±

0.05 0.05 0.05 0.05

film thickness measured by XRR (nm)

film thickness measured by ellipsometry (nm)

± ± ± ±

20.3 10.5 8.6 8.5

21.9 11.6 9.0 9.3

0.1 0.1 0.1 0.1

film electron densities obtained from XRR data fitting and film thicknesses measured using both XRR and ellipsometry for the four MLD films investigated in this work. For all of the MLD films, good agreement between XRR and ellipsometric thickness was achieved, validating the ellipsometric film thickness values. It is also clearly evident that the cross-linked polyurea MLD films have higher film densities than the noncross-linked film. While the non-cross-linked DICB/ED film has a normalized electron density of 0.37 ± 0.05, the crosslinked DICB/DETA, DICB/TETA, and DICB/TAEA films have normalized electron densities of 0.56 ± 0.05, 0.49 ± 0.05, and 0.56 ± 0.05, respectively. From the non-cross-linked to the cross-linked MLD polyurea films, film density increased by approximately 50%. Moreover, the electron densities of the cross-linked polyurea MLD films are similar to those of crosslinked polyamide thin films grown by interfacial polymerization, with electron densities between 0.54 and 0.57 (normalized to silicon),42 and those of the cross-linked poly(ethylene glycol)like films deposited by plasma enhanced chemical vapor deposition, which have electron densities between 0.54 and 0.56 (normalized to silicon),43 demonstrating the capability of MLD of forming high quality thin films. The XRR data can also be used to provide other information on the film properties. For example, XRR analysis yields film roughness in addition to thickness and electron density. For the DICB/ED, DICB/DETA, DICB/TETA, and DICB/TAEA films, the film roughness was measured as 0.46 ± 0.01, 0.39 ± 0.01, 0.38 ± 0.01, and 0.45 ± 0.01 nm. However, the difference in the film roughness may not be significant enough to suggest a strong impact of cross-linking on film morphology. In addition, the molecular volume of each repeating unit of the polymer films can be approximated from the corresponding electron densities obtained in the XRR measurements. By defining a repeating unit as one DICB plus the corresponding stoichiometric amount of the multiamine, and using the number of electrons per repeating unit and the absolute electron density from XRR, the volume of each polymeric repeat unit can be extracted. Using this analysis, the molecular volumes are determined as 417, 287, 335, and 293 Å3 for the DICB/ED, DICB/DETA, DICB/TETA, and DICB/TAEA films, respectively. The smaller molecular volumes of the

compositions of the cross-linked MLD films both measured by XPS and calculated from theoretical reaction stoichiometry. It can be seen that the experimental values agree very well with theoretical expectations, indicating that the MLD films were deposited with stoichiometric compositions. Since introducing cross-linking to organic films can enhance their film densities, the densities of the MLD polyurea thin films were investigated using XRR, and a comparison between the non-cross-linked and cross-linked polyurea films was performed. Here the electron density normalized to that of the underlying Si substrate was used to evaluate film density, due to the difficulty in deriving film mass density from electron density without an accurate molecular structure of the polyurea films. Therefore, the electron densities reported here are relative values compared to the Si substrate. Figure 3 shows a representative reflectivity curve of a DICB/TETA film, and the curves for the other polyurea films can be found in Figure S1

Figure 3. XRR scan (open circles) of a DICB/TETA film with the StochFit fit (red line). The inset shows the electron density profile (black points) with a box model fit (red line). 5641

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the polymeric thin films. All cross-linked polyurea MLD films exhibit constant growth rates. They also contain the characteristic vibrational modes of urea linkages as confirmed by FTIR spectroscopy. XPS studies show that the cross-linked polyurea films are deposited with stoichiometric compositions which closely match the anticipated atomic ratios. The film density investigation using XRR suggests that the cross-linked films are approximately 50% more dense than the analogous non-crosslinked film. These cross-linked MLD films are not only much more dense but also more thermally stable. As vacuum annealing results show, the temperature at which complete decomposition or desorption occurred was increased from 260 °C for the non-cross-linked polyurea film to above 290 °C for the cross-linked film. This approach of fabricating cross-linked organic thin films can also be extended into other types of polymeric films, for instance, polyamide and polyimide films.

DICB/DETA, DICB/TETA, and DICB/TAEA polyurea films are consistent with the presence of cross-linking, since crosslinking introduces denser films with correspondingly less volume for each repeating unit. Evaluation of the thermal stabilities of the cross-linked polyurea thin films was performed by vacuum annealing the MLD films at different temperatures for 20 min under a 50 mTorr nitrogen flow. For each of the polyurea MLD films, the samples used in the annealing experiments were deposited in the same batch with little film thickness variation between different samples. FTIR spectra of the annealed samples were taken after annealing and used to assess the remaining chemical integrity of each sample, where the absorbance of vibrational modes can be used as an indicator of the quantity of polyurea film remaining after annealing. The intensities of characteristic urea absorption bands were monitored, and the peak areas from 1720 to 1420 cm−1 were integrated and used to assess how much film remained after annealing. Figure 4 shows the

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ASSOCIATED CONTENT

S Supporting Information *

XRR curves and data fittings, together with electron density profiles of DICB/ED, DICB/DETA, and DICB/TAEA films. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (S.F.B.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge support of this work from Intel Corporation. We thank Thomas Brennan and Dr. Han-Bo-Ram Lee for valuable discussions. Portions of this work were carried out at the Stanford Synchrotron Radiation Lightsource, a Directorate of SLAC National Accelerator Laboratory, and an Office of Science User Facility operated for the U.S. Department of Energy Office of Science by Stanford University. Dr. Chad Miller is thanked for help with the XRR experiments and analysis of the reflectivity data. S.F.B. acknowledges support from the National Science Foundation (CHE 1213879).

Figure 4. Normalized IR absorbance between 1720 and 1420 cm−1 after annealing at different temperatures of (a) DICB/ED, (b) DICB/ DETA, (c) DICB/TETA, and (d) DICB/TAEA polyurea MLD films.

normalized peak areas of the annealed MLD films as a function of annealing temperature. It can be seen that the cross-linked polyurea films have higher thermal stability than the non-crosslinked DICB/ED film, since the cross-linked polyurea films reach a state of full decomposition or desorption (these cannot be distinguished with the available methods) at higher temperatures than does the non-cross-linked one. For the non-cross-linked DICB/ED film, nearly complete film loss was observed after annealing at 260 °C. In comparison, for the cross-linked DICB/TAEA film, only 40% of the film was lost at 260 °C, and remarkably about 10% of the film was still present after annealing at 290 °C. These results show that cross-linking of the MLD polymer chains results in improvement in the thermal stability of the MLD films.

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REFERENCES

(1) Richert, L.; Engler, A. J.; Discher, D. E.; Picart, C. Biomacromolecules 2004, 5 (5), 1908−1916. (2) Chan, K.; Gleason, K. K. Langmuir 2005, 21 (19), 8930−8939. (3) Yoon, M. H.; Yan, H.; Facchetti, A.; Marks, T. J. J. Am. Chem. Soc. 2005, 127 (29), 10388−10395. (4) Sakai, Y.; Sadaoka, Y.; Matsuguchi, M. Sens. Actuators, B 1996, 35 (1−3), 85−90. (5) Geise, G. M.; Lee, H. S.; Miller, D. J.; Freeman, B. D.; Mcgrath, J. E.; Paul, D. R. J. Polym. Sci., Polym. Phys. 2010, 48 (15), 1685−1718. (6) Qian, H.; Li, S.; Zheng, J.; Zhang, S. Langmuir 2012, 28 (51), 17803−17810. (7) Winkleman, A.; Perez-Castillejos, R.; Lahav, M.; Narovlyansky, M.; Rodriguez, L. N. J.; Whitesides, G. M. Soft Matter 2007, 3 (1), 108−116. (8) Al Akhrass, S.; Gal, F.; Damiron, D.; Alcouffe, P.; Hawker, C. J.; Cousin, F.; Carrot, G.; Drockenmuller, E. Soft Matter 2009, 5 (3), 586−592. (9) Johnson, P. M.; Yoon, J.; Kelly, J. Y.; Howarter, J. A.; Stafford, C. M. J. Polym. Sci., Polym. Phys. 2012, 50 (3), 168−173. (10) Chan, E. P.; Lee, J. H.; Chung, J. Y.; Stafford, C. M. Rev. Sci. Instrum. 2012, 83, 11.

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CONCLUSIONS Cross-linked organic polymeric thin films have many desirable properties compared to non-cross-linked ones, such as higher film density and better thermal stability. In this study, a onestep process for depositing cross-linked polyurea thin films has been demonstrated using a molecular layer deposition (MLD) technique. In contrast to the non-cross-linked MLD films deposited with conventional combinations of bifunctional precursors, the employment of tri- and tetrafunctional precursors can introduce cross-linking and bridging sites into 5642

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dx.doi.org/10.1021/ma400998m | Macromolecules 2013, 46, 5638−5643