J. Phys. Chem. C 2010, 114, 14843–14848
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Kinetics of Stop-Flow Atomic Layer Deposition for High Aspect Ratio Template Filling through Photonic Band Gap Measurements Siva Krishna Karuturi,† Lijun Liu,† Liap Tat Su,† Yang Zhao,† Hong Jin Fan,‡ Xiaochen Ge,§ Sailing He,§ and Alfred Tok Iing Yoong*,† School of Materials Science and Engineering, Nanyang Technological UniVersity, 50 Nanyang AVenue, 639798 Singapore, DiVision of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological UniVersity, 21 Nanyang Link, 637371 Singapore, and Centre for Optical and Electromagnetic Research, Zhejiang UniVersity, Hangzhou, 310058 P. R. China ReceiVed: June 11, 2010; ReVised Manuscript ReceiVed: July 14, 2010
Atomic layer deposition (ALD) is shown as a unique method to produce high aspect ratio (AR) nanostructures through conformal filling and replication of high AR templates. The stop-flow process is often used as an alternative to the conventional continuous flow process to obtain high step coverage. However, there is a need for understanding the deposition kinetics and optimizing the deposition process to fabricate defect-free nanostructures. In this Article, TiO2 ALD in high AR self-assembled opal photonic crystal templates was performed in stop-flow fill-hold-purge process in comparison with continuous flow pulse-purge process. Photonic band gap properties of opal templates were characterized and compared with simulated band diagrams for quantitative investigation of filling kinetics and the effect of shrinking pore size on filling uniformity. Γ-L bands in the transmittance spectra of ALD-infiltrated opals accurately represented the depth profile of the depositions without the need for expensive sample preparation techniques and characterization tools. It was found that the stop-flow process attains higher Knudsen flow rates of precursor gases, thereby achieving homogeneous and complete filling at considerably lower cycle time. 1. Introduction Atomic layer deposition (ALD) has evolved as a unique tool for nanotechnology with atomic level control of 3D conformality and homogeneity. Film depositions are realized for complex nonplanar topographies, such as nanowires, nanobowls, nanotubes, and 2D and 3D photonic crystals.1-4 Some of these realizations are nanodevices for energy conversion and storage, nanoparticle catalysts, nanostructures for drug delivery, gas separations, sensing, and photonic applications.5-13 Functional properties of the obtained nanostructures such as electrical properties of nanowires and nanotubes and optical properties of photonic crystals are critically influenced by the depth uniformity of ALD deposition. ALD in extremely complicated surfaces with high aspect ratio (g103) has recently received great attention in various disciplines. However, deposition on complicated surfaces is more difficult than that over flat surfaces; in the former case, a nonuniform degree of saturation in the nanostructures is generally produced, leading to a thickness gradient from the top surface to the bottom. Understanding the mechanism and control of ALD in high aspect ratio (AR) substrates is not straightforward because of the difficulty of direct access to the deeper surfaces during or after the deposition. The George group used in situ conductance measurements and ex situ electron probe microscopy for mesoporous tubular membranes and anodic alumina, respectively, to investigate the high AR deposition kinetics.14,15 The Gordon group proposed a simple and effective model based on * Corresponding author. E-mail:
[email protected]. Tel: (65) 91522767. † School of Materials Science and Engineering, Nanyang Technological University. ‡ School of Physical and Mathematical Sciences, Nanyang Technological University. § Zhejiang University.
Knudsen molecular flow to establish the kinetic requirements of ALD with respect to the structural parameters of the nanostructures.16 They recently performed ex situ Rutherford backscattering spectrometry for the depth profile analysis of silica aerogel monoliths to explore the mechanism of deposition in ultra-high AR nanoporous solids.17 However, all previous studies using high AR templates focused only on coating of a few nanometers to modify the original surfaces. No reports so far are available on detailed ALD process optimization for high AR template replication to account for shirking pore size with the progress of the deposition. Filling highly porous nanostructures is an important technological function of ALD for many applications in nanoelectronics, nanophotonics, and nanobio fields.13,18-21 Highly precise filling can significantly improve the robustness and reliability and provide great control over the functional properties. Replication of high AR templates with complete filling necessitates stringent kinetic requirements due to the continual increase in AR (or shrinkage of pore opening) with the progress of deposition. ALD operated in continuous flow process (pulse-purge) requires very long exposure times and gives lower control for depositions in high and ultra-high ARs. Therefore, stop-flow ALD processes are believed to be suitable for high AR substrates.22-24 Figure 1 shows the major difference between stop-flow and continuous flow processes. In the stopflow process, the pulse step is divided into fill and hold, where the precursor gases are filled up to the set pressures and held at this pressure. Whereas the ability to control the precursor exposure time is unchanged, the additional fill step builds up the chamber pressure and thus enables the deposition at higher precursor partial pressures, which is beneficial particularly to depositions on 3D nanostructured surfaces. Whereas this process is available in most commercial ALD systems, critical evaluation
10.1021/jp1053748 2010 American Chemical Society Published on Web 08/16/2010
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Figure 1. Comparison of one cycle of ALD process for continuous flow (solid line) and stop flow (dashed line).
of the kinetics of stop-flow processes for high AR depositions is still missing. In this Article, we systematically investigate the effect of various process parameters on the uniformity of TiO2 ALD in the porous framework of opal photonic crystals. Depositions are performed in both continuous flow process and stop-flow process for comparison. We measure transmittance spectra of the infiltrated opals to determine photonic band gaps (PBGs) of the ALD-infiltrated opals. We compare width and position of the PBGs with simulated band structures to understand the depth profile of the depositions and correlate with the filling kinetics without the necessity of ex situ characterizations. This study also provides useful information for depositions in other 3D templates such as deep holes or trenches, aerogels, and anodic aluminum oxide membranes. 2. Experimental Methods 2.1. Materials and Deposition Method. Self-made viscous flow reactor type of ALD was used in this study, as explained elsewhere.25 Titanium tetrachloride with 99.999% purity purchased from Sigma Aldrich and deionized water were used as precursors for TiO2 deposition. Carrier gas precursor flow rates of 4:1 and 8:1 were used for H2O and TiCl4, respectively. Depositions were carried out at a constant temperature of 70 °C. Carboxyl-modified, monodispersed polystyrene particles (510 nm diameter) of 10% solids content were bought from Duke scientific corporation for opal template preparation. For high AR ALD, the opal template thickness was controlled at 30 µm, and a relatively small surface area of 1 × 1 cm2 free from imperfections was cleaved. It is to be noted that formation of cracks is inevitable during the fabrication of multilayer opal templates.26 In our experiment, the crack island sizes were around 100 × 100 µm2. This leaves at least 70% of the total area of opal crystal free from the lateral transport of the precursor molecules through crack openings. Glass was chosen for substrate because its transparent nature in the wavelength ranges under consideration aids the transmission measurements. 2.2. Characterizations and Measurements. Field emission scanning electron microscopy (JEOL JSM 6340F) was used to obtain high-resolution microstructural images of the opal templates. A UV-vis-NIR Cary 5000 spectrophotometer (from Varian) was used to measure specular reflectivity and transmittance data. Transmittance measurements were carried out at zero incidence angle. Refractive index values for TiO2 films were obtained from reflectivity and spectral ellipsometry measurements.
Karuturi et al. 3. Results and Discussion 3.1. Opal-Based Characterization of Deposition Uniformity. Opal templates were prepared by self-assembly of monodispersed polystyrene spherical particles in face-centered cubic arrangement using vertical self-assembly technique (Figure 2).27 Because of the highly periodic arrangement of polystyrene spheres, opals are 3D photonic crystals exhibiting PBGs in the (111) direction, where propagation of light within certain frequency ranges is prohibited. Positions and widths of the PBGs are dictated by the size and the dielectric constant of the spheres once a close packed structure is fixed.28 PBGs were experimentally determined by the transmittance or reflectance measurements in the (111) direction using a spectrophotometer. Theoretical band diagrams were calculated by the plane wave expansion method using the freely available MIT photonic bands (MPB) package. Figure 3a shows the measured transmittance spectrum (lower part) and simulated band diagram (upper part) for opal templates. When voids of the opals are infiltrated with ALD of TiO2, PBGs from infiltrated opals arise at longer wavelengths because of the increased average dielectric constant. Figure 3b shows the theoretically calculated bandwidth for opals (510 nm sphere size) with different filling fractions of the TiO2. It can be observed that the position of the PBG shifts to longer wavelengths continuously with the increase in filling fractions. The width of the band gap gradually decreases to a point where it completely disappears as the dielectric constant of the background (which can be approximated by averaging the dielectric constants of TiO2 and the remaining air) of the photonic crystal becomes equal to that of the polystyrene spheres and then increases gradually with further filling. Therefore, by examining the transmittance of the infiltrated template in (111) plane and comparing Γ-L band with simulated band diagrams, we can calculate the filling fractions of ALD-infiltrated materials. In this study, a fixed number of ALD cycles (400) was used in all experiments to achieve maximum possible filling of the opals. In the case of nonuniform ALD filling, the filling fraction varies with the depth, and the maximum filling always occurs at the top surface, whereas the lowest filling (fb) always occurs in deepest layers near the substrate. Band gaps in the measured transmittance spectra of nonuniformly deposited templates consist of broadened and multiple peaks due to the overlapping of peaks with different filling fractions from different depths. Figure 3c shows the overall width of band gap with respect to the extent of nonuniformity of the infiltrated opals. Therefore, by following the width of the band gap peaks in the transmittance spectra of infiltrated opal, it is possible to understand the depth profile information; we are thereby able to study filling kinetics using PBG measurements as a facile characterization method. Aspect ratios (ratio of longest dimension to the smallest dimension) of the ALD-deposited opal templates were calculated to generalize and correlate the filling kinetics with structural parameters of the substrate. Longest dimension was considered to be three times the thickness of the opal structure (30 µm) because curvaceous channels in the opal structure present a situation of tortuous flow of precursor gases.29 The shortest dimension was calculated as the diameter of the spherical opening of area equal to the area between spheres in the closedpacked plane. With the progress of deposition on the spheres, pore size shrinks continuously. We performed Monte Carlo simulations to calculate deposited shell thickness and the reduced pore area to determine the shortest dimension corresponding to the filling fraction, fb. Therefore, calculated aspect ratio based on the lowest filling fraction fb (Figure 3d) corresponds to the
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Figure 2. FESEM images of opal templates used for ALD infiltration: (a) cross sectional view and (b) surface view.
Figure 3. (a) Transmittance spectrum of opal templates used for ALD infiltration (lower part) and simulated band diagram (upper part). (b) Simulated band diagram at L for infiltrated opal templates with different filling fractions of TiO2, showing closing and reopening of the band gap. (c) Simulated width of the band gap versus lowest filling fraction fb of the nonuniformly infiltrated template, with the red line indicating the top of the band gap of the structure with filling fraction 1.0 and the black line showing the bottom of the gap varying with different filling fractions. (d) Aspect ratio of the infiltrated opal structure corresponding to the lowest filling fraction fb.
aspect ratio below which deposition occurs uniformly throughout the depths, above which nonuniformity sets up and no further depositions occur at the bottommost layers of the opal structure. 3.2. Continuous Flow (Pulse-Purge) Process. First, we performed ALD in pulse-purge mode by varying the pulse time of TiCl4 (15, 30, 45, and 60 s) while all other experimental parameters were kept constant. A fixed purge time of 60 s was used for both TiCl4 and H2O. Pulse time of H2O was kept at 30 s. The partial pressure of H2O was set four times higher than that of TiCl4. Pressure in the reaction chamber during the deposition was around 0.35 mbar. It can be assumed that ALD half reaction during TiCl4 pulse is the limiting factor to the deposition uniformity by considering the much higher exposure (partial pressure and exposure time) and molecular diffusion of H2O in comparison with TiCl4. Therefore, to determine the effect of TiCl4 pulse time on the uniformity of depositions, transmittance measurements of TiO2-infiltrated opal templates were carried out. As shown in Figure 4a, the Γ-L band gap in the transmission spectra for the infiltrated opal with 15 s pulse time is very broad, spanning all the way from full infiltration
at the top (∼1350 nm) to lowest infiltration at the bottom (∼1100 nm), indicating highly nonuniform deposition. For infiltrated opals with all of the exposure times (15, 30, 45, and 60 s), infiltration was insufficient in the deeper layers, and improvement by increasing the pulse times up to 60 s was still not good enough to realize uniform full infiltrations. The lowest filling fractions (fb) in the deepest opal layers were calculated using the width of the PBGs from the transmission spectra and converted into aspect ratios, as shown in Figure 4b. When the ARs for different pulse times are compared, it can be noticed that the AR is increased by almost square root of two times from 820 to 1170 when the pulse time was increased by two times from 30s to 60s, showing square root dependence of aspect ratios on exposure time as per the Knudsen diffusion theory.16 In addition, it can be observed that all transmission spectra do not consist of multiple peaks, except the discontinuity at ∼1250 nm, which is due to index matching phenomena, as explained in Section 3.1, indicating graded deposition. Deposition occurred uniformly in the beginning with sufficient exposure of titanium chloride (for pulse time >30 s).
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Figure 4. ALD in continuous flow (pulse-purge) process with varying pulse times of TiCl4: (a) transmission spectra and (b) filling fraction at the bottom and aspect ratio.
TABLE 1: Parameters Used for the Kinetic Study of Opal Infiltration in Stop-Flow Process experiment type effect of fill pressure effect of hold time effect of purge time effect of opal thickness
precursor
fill pressure, [mbar]
hold time, [s]
purge time, [s]
H2O TiCl4
4 0.5, 4, 8,12, 16
20 20
60 60
TiCl4
8
60
TiCl4
12
10, 20, 30, 40,50 20
TiCl4
2
10
30, 45, 60, 75, 90 60
As the deposition thickness of titanium dioxide on the surface of the polystyrene spheres increases with deposition progress, the smallest dimension (pore opening) shrinks further to cause an increase in aspect ratio, making it more difficult for the precursor molecules to diffuse into the deeper section of the substrate. Deposition followed the graded nature subsequently. This type of deposition evolution further confirmed that insufficient Knudsen diffusion of TiCl4 was the main cause for nonuniformity. The continuous flow process requires ∼700 s pulse time according to the Knudsen diffusion theory (calculated from the square root dependence of AR on exposure time) to achieve 96% of the filling uniformly, where the AR of the substrate becomes 4000 for the templates used in this study. (See Figure 3d.) On the basis of similar exposure requirements for the other precursor, the total cycle time becomes too long for practicable deposition. Half life and reversibility of chemical reactions for several precursors may possibly worsen the quality of deposition as well.30 Increasing the partial pressure of the precursors by either lowering the evacuation rate to increasing the total pressure in the reactor (residence time of gases) or increasing the partial pressures of the precursor using high-temperature precursor vaporization may enhance aspect ratios further, but, the open vacuum method of operation makes it very difficult to scale the exposures required for high AR filling. Also, higher flow rates of corrosive precursor gases such as TiCl4 in continuous flow process may have a detrimental effect on several main components of the ALD equipment. 3.3. Stop-Flow (Fill-Hold-Purge) Process. The fillhold-purge method, as introduced in the previous section, was used to infiltrate the opal templates with the aim of achieving uniform filling and lowering the cycle time. The effect of various process parameters on deposition uniformity was studied systematically for titanium chloride, as shown in Table 1. Optimized exposure (row 1 in Table 1) for water precursor was used commonly in all experiments so as to ensure that the ALD half reaction during the TiCl4 pulse is the limiting factor to deposition uniformity. First, we infiltrated opal templates by varying the fill pressure for TiCl4 while keeping all other parameters constant (row 2 in
Table 1). It can be seen from Figure 5a that PBGs of the infiltrated opals with different fill pressures are smaller in width when compared with those of continuous flow process (Figure 4a), showing better control in high aspect ratio filling. Uniformity of the deposition gradually improved with fill pressures from 0.5 to 8 mbar, but the trend reversed when the fill pressure was increased further. Figure 5b shows that when 8 mbar was used as fill pressure for titanium chloride, uniformity of the deposition enhanced greatly. The bottommost layers of the opals were infiltrated with 96% of the maximum possible filling. Also, AR of the deposition for 8 mbar fill pressure is increased by almost 4 times when compared with that of 0.5 mbar. The square root dependence of aspect ratio on precursor pressures is in accordance with the Knudsen diffusion theory.16 However, AR of the deposition is expected to increase continuously with increasing pressure. The chances of deterioration of deposition uniformity due to the reversibility of ALD reactions at higher filling pressures is negligible because the cycle time remains almost the same for all experiments. One possible cause is that the deposition occurred outside the self-limiting reaction window of ALD. The lower deposition temperature (70 °C) used in the present study to protect the thermally sensitive polymer template might cause the precursor gases to condense on the substrate with weak physical interactions rather than the required single layer adsorption. If true, this would change all deposition behavior during the next steps, leading to the nonuniformity. To rule out such possibility, we conducted depositions at 50 °C on flat substrates under all of the same experimental conditions. The results showed that the growth rate was ∼0.99 Å per cycle, which is very much in accordance with self-limiting ALD growth. Some of the recent work of TiO2 ALD at low temperatures on thermally sensitive substrates also corroborates our conclusion.11,31 Hence, the thermal effect on the deposition being eliminated, the cause for reversible trend with fill pressures higher than 8 mbar, can be speculated as a possible occurrence of unwanted chemical vapor deposition (CVD). Increasing AR with the progress of the deposition makes it increasingly difficult for the excess precursor gases to escape from the internal pore volume within the same purge time, resulting in CVD reactions in the midst of the infiltration. This type of unwanted CVD deposition is expected to cause the premature blocking of precursor transfer paths and halting of further deposition in the deeper layers. Moreover, the transmission spectrum for the highest fill pressure of 16 mbar consists of multiple peaks against the graded evolution at insufficient exposures, indicating the CVD nature of the deposition. To test the hypothesis that unwanted CVD reactions caused the deposition nonuniformity in stop-flow process at higher precursor exposures, we carried out further filling experiments by varying the hold time and purge time of titanium chloride precursor, as shown in Table 1. We gradually varied hold time
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Figure 5. ALD in stop-flow (fill-hold-purge) process with varying fill pressures for TiCl4: (a) transmission spectra and (b) filling fraction at the bottom and aspect ratio.
Figure 6. Filling fractions at the bottom and aspect ratios of the stop-flow process as a function of (a) TiCl4 hold time and (b) TiCl4 purge time.
of TiCl4 precursor from 10 to 50 s by keeping all other parameters constant. As shown in Figure 6a, an optimum hold time of 20 s can be determined. Exposure times shorter than 20 s produced lower infiltration, which can result from the insufficient amount of Knudsen gas flow. Exposure times of 30 s or longer worsened the uniformity, showing a similar tendency with fill pressure. As for the control experiments with different purge times, the result was shown in Figure 6b. In our experiment, filling fractions and AR of ALD increased continuously all through the purge times considered. An optimum purge time of 75 s can be determined. Therefore, experiments with varying hold time and purge time of TiCl4 confirm that the occurrence of CVD reactions due to insufficient purge times is the reason for nonuniformity at higher exposures. Therefore, the purge step in high AR deposition must be designed to meet the time required for excess precursor mass transport out of the deep holes. The total cycle time for stop-flow process (filling time + hold time + purge time) required for uniform and full infiltration (>96%) of the opals was almost an order of magnitude less than that of the continuous flow process (pulse time + purge time). Therefore, it can be understood that whereas increasing the exposure time in the continuous flow process improves the homogeneity of the high AR deposition (Figure 4b), it offers a less practicable way when compared with controlling the fill pressure in the stop-flow process. The latter reduces the cycle time greatly and meets the more stringent kinetics required for high AR deposition (Figure 5b). Finally, filling experiments were carried out for opals with different thickness to test the accuracy of photonic-based characterizations. Depositions were carried out in stop-flow processes in the Knudsen diffusion limiting regime. Figure 7 shows that equal AR values (variation 5%) were obtained for all templates when deposited under the same process conditions. This also confirms the photonic crystal-based methodology used to quantify ALD for high AR opal templates is reasonably accurate enough.
Figure 7. Filling fractions at the bottom and aspect ratios of the stopflow process as a function of opal thickness.
4. Conclusions High-quality opal photonic crystal templates with dimensional control were prepared and used for the investigation of the filling kinetics of stop-flow ALD. Depositions were performed in both stop-flow and continuous flow processes for comparison. PBGs of infiltrated opal templates from the transmittance spectra were compared with simulated band diagrams to evaluate the depth profile. Within the experimental error, photonic-based characterizations accurately quantified the filling kinetics. Continuous flow processes needed impractical lengths of exposure time for achieving full and uniform fillings because of the insufficient Knudsen flow of precursor gases. The stopflow process made it easier to achieve high AR depositions with cycle times of an order of magnitude less when compared with that of the continuous flow process. Depth-uniform deposition with AR of ∼4000 was achieved with highly precise spatial deposition control in stop-flow process. Therefore, it is recommended to use the stop-flow process for high aspect ratio template filling, especially for unstable precursors, which decompose at longer exposure times. In addition, kinetics studies showed that uniform infiltrations require an optimum process window for other parameters such as purge time to eliminate the occurrence of unwanted CVD reactions.
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Acknowledgment. This work was supported by the Ministry of Education, Singapore by Tier 2 Academic Research Fund under grant numbers T208A1225 and T207B1214. References and Notes (1) Ritala, M.; Leskela¨, M. Nanotechnology 1999, 10, 19. (2) Knez, M.; Nielsch, K.; Niinisto¨, AdV. Mater. 2007, 19, 3425. (3) Leskela¨, M.; Kemell, M.; Kukli, K.; Pore, V.; Santala, E.; Ritala, M.; Lu, J. Mater. Sci. Eng., C 2007, 27, 1504. (4) Kim, H.; Lee, H. B. R.; Maeng, W. J. Thin Solid Films 2009, 517, 2563. (5) Nanu, M.; Schoonman, J.; Goossens, A. AdV. Mater. 2004, 16, 453. (6) Banerjee, P.; Perez, I.; Henn-Lecordier, L.; Lee, S. B.; Rubloff, G. W. Nature Nanotechnol. 2009, 4, 292. (7) King, J. S.; Wittstock, A.; Biener, J.; Kucheyev, S. O.; Wang, Y. M.; Baumann, T. F.; Giri, S. K.; Hamza, A. V.; Baeumer, M.; Bent, S. F. Nano Lett. 2008, 8, 2405. (8) Alessandri, I.; Zucca, M.; Ferroni, M.; Bontempi, E.; Depero, L. E. Small 2009, 5, 336. (9) King, J. S.; Galliot, D. P.; Graugnard, E.; Summers, C. J. AdV. Mater. 2006, 18, 1063. (10) Fan, H. J.; Knez, M.; Scholz, R.; Nielsch, K.; Pippel, E.; Hesse, D.; Zacharias, M.; Go¨sele, U. Nat. Mater. 2006, 5, 627. (11) Knez, M.; Kadri, A.; Wege, C.; Go¨sele, U.; Jeske, H.; Nielsch, K. Nano Lett. 2006, 6, 1172. (12) Peng, Q.; Sun, X. Y.; Spagnola, J. C.; Hyde, G. K.; Spontak, R. J.; Parsons, G. N. Nano Lett. 2007, 7, 719. (13) Kim, G.-M.; Lee, S.-M.; Michler, G. H.; Roggendorf, H.; Go¨sele, U.; Knez, M. Chem. Mater. 2008, 20, 3085. (14) Elam, J. W.; Routkevitch, D.; Mardilovich, P. P.; George, S. M. Chem. Mater. 2003, 15, 3507. (15) Berland, B. S.; Gartland, I. P.; Ott, A. W.; George, S. M. Chem. Mater. 1998, 10, 3941.
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