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Poly(cyclohexylethylene)-block-Poly(lactide) Oligomers for Ultrasmall Nanopatterning Using Atomic Layer Deposition Li Yao,† Luis E. Oquendo,† Morgan W. Schulze,‡ Ronald M. Lewis, III,‡ Wayne L. Gladfelter,*,† and Marc A. Hillmyer*,† †
Department of Chemistry and ‡Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455-0431, United States S Supporting Information *
ABSTRACT: Poly(cyclohexylethylene)-block-poly(lactide) (PCHE−PLA) block polymers were synthesized through a combination of anionic polymerization, heterogeneous catalytic hydrogenation and controlled ring-opening polymerization. Ordered thin films of PCHE−PLA with ultrasmall hexagonally packed cylinders oriented perpendicularly to the substrate surface were prepared by spin-coating and subsequent solvent vapor annealing for use in two distinct templating strategies. In one approach, selective hydrolytic degradation of the PLA domains generated nanoporous PCHE templates with an average pore diameter of 5 ± 1 nm corroborated by atomic force microscopy and grazing incidence small-angle X-ray scattering. Alternatively, sequential infiltration synthesis (SIS) was employed to deposit Al2O3 selectively into the PLA domains of PCHE−PLA thin films. A combination of argon ion milling and O2 reactive ion etching (RIE) enabled the replication of the Al2O3 nanoarray from the PCHE−PLA template on diverse substrates including silicon and gold with feature diameters less than 10 nm. KEYWORDS: block polymer, self-assembly, nanolithography, nanopores, atomic layer deposition, selective infiltration synthesis, solvent vapor annealing
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INTRODUCTION Block polymer nanolithography produces high density nanostructured arrays for a broad range of technologies, including microelectronics,1−5 photovoltaics6−9 and high density magnetic data storage.10−15 Block polymers adopt well-defined morphologies determined by the overall degree of polymerization (N), the volume fraction of each block (f) and the block incompatibility (χ).16 The formation of ultrahigh density arrays is dictated by the segregation strength (χN) such that block polymer systems of larger χ are required for self-assembly as N is reduced to achieve ultrasmall features.17,18 Poly(styrene)-block-poly(methyl methacrylate) (PS− PMMA) has been widely investigated as a template for block polymer lithography, due in part to the similar interfacial energies of the two blocks that facilitate the formation of features oriented perpendicular to the substrate surface.19−23 We previously developed poly(cyclohexylethylene)-block-poly(methyl methacrylate) (PCHE−PMMA) as a “high-χ” alternative to PS−PMMA.24 Readily synthesized by catalytic hydrogenation of polystyrene (PS), poly(cyclohexylethylene) (PCHE) is an amorphous hydrocarbon with a higher glass transition temperature and increased resistance to oxidative degradation compared to PS. Most importantly, PCHE is more incompatible with hydrophilic blocks than PS, enabling the formation of sub-10 nm features in PCHE−PMMA.24 © XXXX American Chemical Society
However, poor etch resistance and the low etch contrast between the two organic blocks can be disadvantageous in nanopatterning applications.25,26 Appropriate block polymer design is needed to leverage orthogonal removal strategies that can enable high fidelity pattern transfer.27 We previously demonstrated the generation of robust nanoporous PCHE monoliths from ordered poly(cyclohexylethylene)-block-polylactide (PCHE−PLA) block polymers following the selective removal of the polylactide block by simple hydrolysis.28 Herein, we prepare low molar mass block oligomers of PCHE-PLA with higher χ values than PCHE−PMMA to generate thin films with ultrasmall domains. Two approaches used in this study to generate multifunctional templates for thin film patterning from PCHE−PLA are shown in Scheme 1. In one route, a nanoporous PCHE template is produced from a self-organized thin film composed of hexagonally packed cylinders oriented perpendicular to a Si substrate after the selective hydrolytic decomposition of the PLA domains. Reactive ion etching (RIE) and/or Damascenetype processes can then be used to transfer the pattern of the nanoporous template into the Si wafer.10,27,29,30 In this work, Received: December 30, 2015 Accepted: February 18, 2016
A
DOI: 10.1021/acsami.5b12785 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
styrene (PSOH) was hydrogenated to PCHEOH using Pd/ CaCO3 at 120 °C for 2−3 d; a process known to result in only minor loss of functionality at the hydroxyl-terminus. 18 Combining PCHEOH and DBU leads to the controlled ringopening polymerization of lactide from PCHE−OH. Samples of varied molar mass (2.9−11.3 kg/mol) and composition ( f PLA = 0.34−0.55) were prepared (Table S1). Samples are denoted as PCHE−PLA(X, Y), where X is the number-average molar mass of the block polymer in kg mol−1 and Y is the volume fraction of PLA as determined by 1 H NMR spectroscopy. Previously, Wolf and Hillmyer28 used solubility parameter data and the principal domain spacing of a lamellarforming PCHE−PLA in the strong-segregation regime to estimate the interaction parameter, χ, and predicted an approximate 3-fold increase over the related PS−PLA.41 Here, we estimate χ(T) from measurement of the isochronal linear dynamic moduli upon heating for six near-symmetric ( f PLA = 0.47−0.55) block polymers with thermally accessible order-to-disorder transitions. TODT was determined as the temperature at which the dynamic elastic modulus (G′) shows a marked drop over a small temperature range (Figure S5a). According to mean field theory, the value of the interaction parameter at order−disorder transition (χODT) can be estimated as χODT = 10.5/N, enabling a linear expression for χ in the form χ = α/T + β as
Scheme 1. Illustration of the Dual Functionality of PCHE− PLA Block Polymers in Thin Film Templating Applicationsa
χ 118 =
(222 ± 15) − (0.29 ± 0.03) T
(1)
which after correction to a common reference volume (118 Å3) agrees with previous estimates of χPCHE−PLA ≈ 3χPS−PLA at the cited temperature of T = 187 °C.28,41 A comparison of the PCHE−PLA interaction parameter to those of other block polymers used for nanopatterning is also given in Figure S5b. From these estimates, the large α term results in the largest χ value at low temperature at the cost of an increasing temperature sensitivity. This leads to crossover in the value of χ with other PCHE-systems at high temperatures (T ≳ 227 °C). As a representative sample, we examined the thin film assembly and utility of PCHE−PLA (11.3, 0.34), a block polymer that self-assembles in the bulk into a hexagonally closepacked cylinders of 12 nm diameter and a center-to-center distance of 20 nm (q* = 0.37 nm−1, q/q* = 1, √3, √4, √7, √9) (Figure 1a). PCHE−PLA (11.3, 0.34) was spin-coated as a dilute solution from toluene onto HMDS-treated thermal oxide silicon wafers, resulting in disordered but microphaseseparated films (Figure S7b). Solvent vapor annealing (SVA) has been shown to improve the lateral ordering of block polymer templates leading to highly ordered nanoscopic features at a specified volume of solvent absorbed and/or evaporation rate.31,32,42,43 In contrast to thermal annealing strategies, SVA simultaneously enhances polymer chain mobility and modulates free surface interactions, thereby allowing block−solvent selectivity to dictate a preferred feature orientation by creating an effectively neutral free surface or a surface preferential for one block.44 Here, we utilized a stainless-steel solvent annealing chamber constructed with a dry N2 gas source that delivered solvent vapor at a controlled flow rate, pressure and low humidity to the sample space containing the PCHE−PLA film. In situ spectral reflectancebased measurements of film thickness (Figure S8) obtained through the fused silica viewport of the sample chamber enabled the absorbed volume to be monitored during solvent
a
In one route, a nanoporous PCHE mask is produced from an organized film through hydrolytic decomposition of the PLA domain. Alternatively, the combination of domain selective ALD, argon ion milling and RIE produces an Al2O3 nanoarray.
we characterize PCHE nanoporous thin films with pore sizes as small as 5 ± 1 nm that can be used as templates for pattern transfer to produce ultrasmall nanoarrays.31−33 Alternatively, we explored the use of sequential infiltration synthesis (SIS), an adaptation of atomic layer deposition (ALD) previously described by Elam, Darling and co-workers to deposit Al2O3 selectively in the PLA domain of a PCHE−PLA film to enhance the etch contrast for subsequent pattern transfer applications.34−39 ALD can be used to control the growth thickness with high precision by allowing sequential selective reactions of gas precursors within one domain of the nanostructured thin film.40 The chemical reaction is self-limiting and driven to completion in each cycle by removing the unreacted reagents. In this system, gaseous trimethylaluminum (TMA) selectively infiltrates the PLA block likely due to favorable chemical interactions between the carbonyl moieties and the TMA and then reacts with water vapor in alternating gas pulses producing layers of Al2O3 in the PLA domains. This method in combination with argon ion beam milling and O2 RIE produces well-defined Al2O3 nanoarrays on diverse substrates including silicon and gold with feature diameters less than 10 nm.
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RESULTS AND DISCUSSION PCHE−PLA block polymers were obtained by a combination of anionic polymerization and ring-opening transesterification polymerization techniques previously described (Scheme S1 of the Supporting Information (SI)). Hydroxy-terminated polyB
DOI: 10.1021/acsami.5b12785 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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In situ control of film swelling and post-analysis of the film structure via grazing-incidence small-angle X-ray scattering (GISAXS) and AFM provided insight into the role of the final toluene concentration on the extent of ordering in the dried state. Figure 2 follows the evolution of the film structure upon progressive absorption of toluene followed by rapid solvent removal. At low solvent absorption levels, the resulting film surface is covered with a high density of elongated structures and few circular features. At intermediate concentrations up to 36 vol %, the surface transitions from a mixture of predominantly parallel to predominantly perpendicular cylinders, leading to higher order in-plane reflections of the 2D GISAXS pattern. As shown in the corresponding 1D GISAXS profile (Figure 1a), these scattering maxima are positioned at q/ q* = 1, √3, √4, √7, √9, thus confirming the long-range order of the hexagonal morphology with q* = 0.39 nm−1 in close agreement with the bulk SAXS data. These results allude to the relative contributions of solvent absorption and solvent evaporation rates to thin film ordering.44−46 Here, we utilized a fixed quench rate under progressive extents of solvent absorption. Baruth et al.32 previously demonstrated that under these quenching conditions, the optimal swelling ratio for perpendicular features with minimal defects corresponds to an ordered phase window neighboring, but just below the ODT for block polymers containing PS−PLA. Greater swelling ratios led to a solventswollen disordered state, and poorly ordered films resulted upon drying. Moreover, film dewetting often accompanied high levels of solvent incorporation. In the specific case of PCHE− PLA films, increased solvent concentrations from 42 up to 47 vol % toluene resulted in film dewetting (Figure S9c), a result possibly attributable to the lack of mechanical integrity of a disordered and low molar mass block polymer solution. PCHE−PLA block polymer templates were also easily oriented under less stringent SVA techniques such as a simple “jar annealing”, in which a sample is suspended on a platform above a solvent pool sealed in a glass vessel (Figure S7a). Opening of the vessel vents the system and mimics a rapid quench with the amount of solvent absorbed set by the annealing time. In agreement with the previous SVA, toluene induced a well-ordered arrangement of hexagonally closed packed cylinders (Figure S7d); chloroform vapor led to a parallel orientation (Figure S7c). Solvent selectivity and the vapor pressure of the solvent are two key parameters known to influence microdomain orientation and long-range order.43 We assume chloroform is essentially a nonselective solvent for PCHE−PLA inducing the formation of parallel morphologies upon solvent vapor annealing whereas the anticipated selectivity of toluene for PCHE leads to increases in the long-range order and likely helps induce the formation of hexagonally packed cylinders perpendicular to the substrate.43,47 Under these conditions, the flexibility of solvent annealing to induce perpendicular ordering atop other substrate materials, including a gold surface prepared by electron beam evaporation and hydrophobic silicon wafers formed through modification of the native oxide, was investigated. After spincoating the polymer on each substrate, the samples were subjected to jar annealing using toluene vapor for 10 min. The resulting perpendicular cylinders of Figure S7e,f demonstrate the limited influence of the substrate under SVA and suggest the versatility of PCHE−PLA for patterning on a variety of substrates.
Figure 1. (a) SAXS of bulk PCHE−PLA (11.3, 0.34) and GISAXS of the corresponding films after climate-controlled solvent vapor annealing and hydrolysis of PLA domain to produce nanoporous PCHE. (b) AFM (phase image) and (c) SEM of the PCHE−PLA film on a thermal oxide Si wafer after toluene vapor annealing using secondary and backscattered electron detection modes simultaneously. Scale bars represent 100 nm.
incorporation and evaporation, where the solvent volume fraction, Φsolvent, was estimated in terms of the measured thickness, t, as t − tfilm Φsolvent = solvent + film tsolvent + film (2) For PCHE−PLA (11.3, 0.34), annealing under the dynamic flow of toluene vapor for ca. 5 min typically resulted in approximately 42 vol % toluene absorbed. Subsequent rapid quenching (1−2 s) through the expulsion of solvent vapor under pure N2 purge resulted in a fixed evaporation rate that reproducibly formed well-organized and uniform templates of hexagonally close-packed cylinders. Figure 1 shows the micrographs of the ordered arrays with consistent average feature diameter and center-to-center distance, 11 ± 1 nm and 19 ± 2 nm, respectively, and natural contrast between PCHE (bright) and PLA (dark) domains in the SEM image is depicted. C
DOI: 10.1021/acsami.5b12785 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 2. AFM height images and corresponding 2D GISAXS patterns obtained on solvent-annealed PCHE−PLA (11.3, 0.34) films on thermal oxide Si wafers resulting from absorbed solvent concentrations of 9, 28, 36 and 42 vol % toluene over a period of 70−300 s. Scale bars represent 100 nm. D
DOI: 10.1021/acsami.5b12785 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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√7q, √8q, √10q, √11q, √12q, √13q) upon annealing at 130 °C for 24 h (Figure S11a). Block polymers that adopt a gyroid morphology are of great interest as templates for the fabrication of nanohybrids and nanoporous materials for applications such as solar cells, photonics and catalysis.49−51 A thin film of this polymer annealed at 130 °C for 12 h produced an oriented gyroid morphology by GISAXS (Figure S11b) with similar principal domain spacing of 16.3 nm.52,53 The PLA was removed from this sample by basic hydrolysis to achieve a nanoporous PCHE film (Figure S11c) with a similar scattering pattern and pore diameter of 4.5 nm, indicating minute film shrinkage during the hydrolysis step.54,55 PCHE−PLA is amenable to patterning processes that exploit the facile degradation of the minority PLA block. For both PCHE−PLA (4.4, 0.34) and PCHE−PLA (11.3, 0.34), 2 s of O2 RIE was used to remove any possible surface wetting layer. The exposed films were then immersed in a 0.5 M NaOH in methanol/water (40:60 v/v) solution for 30 min. No change in film dimension or pore collapse was evident in the AFM of the hydrolyzed films for either block polymer template (Figure 3b,c). Furthermore, elimination of the PLA resulted in significant enhancement of the GISAXS intensities relative to the unetched templates (a secondary peak (q/q* = √3) becomes apparent for etched PCHE−PLA (4.4, 0.34), but no change in the positions of the principal peaks was observed (Figure S10) consistent with the stability of the nanoporous film. Combined these results confirm that hydrolytic degradation retains a hexagonal arrangement of nanopores suitable for nanopatterning applications. The templates shown in Figure 3b,c are poised for subsequent pattern transfer processes such as RIE of the underlying substrate or deposition of other materials in the pores. PCHE−PLA is also a promising candidate for selective deposition techniques using an adaptation of atomic layer deposition (ALD) know as selective infiltration synthesis (SIS), which benefits from a breadth of available precursor gases to template a large variety of materials.34−36 Appropriate choice of precursors enables site-specific interaction with the carbonyl moieties of the PLA domain. Through repeated exposure cycles of the metal oxide precursor and water, the PLA domain hardens, enabling a large increase in the etch contrast of the organic blocks. Subsequent removal of the organic material through calcination or oxygen plasma treatment produces a metal oxide pattern with the same lateral dimensions as the initial block polymer template. For demonstration of domain-selective SIS, annealed films of PCHE−PLA (11.3, 0.34) were used to template aluminum oxide (referred to here as nominally Al2O3), which was grown from gas-phase trimethylaluminum precursor and water using an ALD reactor described in our previous work.33,56,57 The process involves an iterative four-step cycle comprising (i) an initial nitrogen purge of the ALD chamber, (ii) subsequent introduction and chemisorption of the metal oxide precursor, (iii) intermediate purge for the removal of residual precursor and byproducts, and finally, (iv) a water vapor pulse. After 100 cycles, normal incidence Ar ion beam milling was performed for 3 min to remove the Al2O3 overlayer and likely some of the polymer scaffold. Additional experiments have shown that this SIS process can be reduced to 25 cycles with only 1 min of Ar ion beam milling to give the same replication pattern as shown in Figure S12 using both silicon wafers and gold substrates. The AFM and SEM images in Figure 4a,b show that the resulting surface after Ar ion beam milling is a cylindrical array, and in
The high incompatibility of PCHE−PLA block polymers enables a reduction in the molar mass needed to produce smaller domains without sacrificing a well-ordered state in thin films. To this end, PCHE−PLA (4.4, 0.34), a cylinder-forming block polymer with a 9 nm domain spacing in the bulk (Figure S10), was employed to increase the feature density with cylinders of 5 ± 1 nm diameters and 12 ± 2 nm center-tocenter distances measured by AFM. The as spin-coated sample produces an ill-defined surface and a mixed orientation of domains, but following SVA, in which 34 vol % toluene was absorbed over a period of 120 s, a well ordered array at the surface was observed by AFM (Figure 3a). Notable surface
Figure 3. AFM (phase image) of (a) PCHE−PLA (4.4, 0.34) thin film after SVA and (b) porous PCHE film after PLA base hydrolysis on thermal oxide Si wafers. Scale bars represent 50 nm. (c) AFM (height image) of the porous PCHE film from PCHE−PLA (11.3, 0.34) after PLA base hydrolysis on a thermal oxide Si wafer. Scale bars represent 100 nm.
defects likely reflect the challenges of pattern fidelity in the low N limit.17,48 The corresponding GISAXS (Figure S10) gives a strong principal scattering peak with a d-spacing of 10 nm, consistent with that determined in the bulk. Therefore, at roughly constant volume fraction, a 2.4-fold reduction in molar mass corresponds to a decrease in cylinder diameter by a factor of 1.9, consistent with highly stretched chains. As an aside, a separate low molar mass sample PCHE−PLA (3.2, 0.44) adopted the gyroid morphology in bulk by SAXS (√3q, √4q, E
DOI: 10.1021/acsami.5b12785 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Image analysis of the AFM and SEM micrographs of the Al2O3 array (Figure 4c,d) provide an estimated Al2O3 feature diameter of 10 ± 1 nm, in agreement with the macroscopic estimates obtained via GISAXS (Figure S14), and a feature height of about 3 nm (Figure S15), confirming that the characteristic periodicity of the template is retained throughout the replication. Figure 5 shows a cross-sectional SEM image of
Figure 5. SEM cross-sectional image of the Al2O3 nanoarray on a native oxide Si wafer. The sample was mounted in a 45° angle and obtained in secondary electron detection mode. The scale bar represents 50 nm.
the Al2O3 nanoarray on a native oxide Si wafer mounted at a 45° angle. Figure S16 shows that high temperature processes (500 °C for 30 min) can also be used to remove the organic template to yield an Al2O3 array. These Al2O3 arrays are suitable hard masks for subsequent nanopatterning applications.
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CONCLUSIONS We addressed the limitations of low incompatibility and typically moderate etch resistance in nanopatterning with block polymer templates through the synthesis of high-χ PCHE−PLA block copolymers amenable to two patterning strategies. Block polymer templates were solvent annealed under toluene vapor to form well-ordered hexagonal arrays of cylinders with diameters as small as 5 ± 1 nm. The removal of the sacrificial PLA domain produced nanoporous PCHE masks compatible with dry lift-off or Damascene-like processes.31,33 A second strategy based on selective infiltration of trimethylaluminum and water precursors into the PLA domain via ALD yielded hexagonal arrays of aluminum oxide nanodots following Ar ion milling and O2 RIE. Combined these two strategies can allow for a broad range of materials to be patterned from a single block polymer template.
Figure 4. (a) AFM height image and (b) SEM image of Al2O3/ PCHE−PLA composite film on thermal oxide Si wafers after ALD and Ar ion mill etching. (c) AFM height image and (d) SEM image of Al2O3 nanoarray after complete removal of PCHE−PLA template by RIE using variable percentages of secondary electron and backscattered electron detection modes simultaneously. Scale bars represent 100 nm.
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EXPERIMENTAL SECTION
Reagents. Styrene (≥99% stabilized), dibutylmagnesium (1 M in heptane), sec-butylithium (1.4 M in hexanes), ethylene oxide (≥99.5%) and 1,8-dizabicycloundec-7-ene (DBU) were purchased from Sigma-Aldrich. Palladium on calcium carbonate (5% Pd) was purchased in reduced form from Alfa Aesar. Cyclohexane and dichloromethane were purified using a solvent system composed of columns of activated alumina and molecular sieves. (±) Lactide was provided by Altasorb, stored under a N2 atmosphere and used as received. Poly(cyclohexylethylene)-block-Poly(lactide) (PCHE−PLA) Synthesis. The polymerization of styrene was performed in cyclohexane at 40 °C for 4−5 h using sec-butyllithium as an initiator.41 End-functionalization was achieved by reaction of living polystyrl carbanions with excess ethylene oxide to yield hydroxyethylated PS. Following termination with methanol, the resulting PSOH was
comparison to the annealed films of Figure 1b is contrast inverted, indicating the formation of impregnated Al2O3/PLA composite domains. Subsequent oxygen plasma treatment removes the organic template, leaving the Al2O3 array. To confirm complete removal, an annealed PCHE−PLA thin film was subjected to O2 plasma etching for 30 s. No features are apparent in the resulting AFM (Figure S13c), which supports the conclusion that complete removal of the organic template is also achieved for the Al2O3/PCHE−PLA composite films. F
DOI: 10.1021/acsami.5b12785 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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film dried within 1−2 s following rapid quench by way of opening of an outlet valve with nitrogen purge. Porous PCHE films were produced by removal of the PLA domain following exposure of the annealed films to a 0.05 M NaOH solution (H2O/CH3OH, 6:4 v/v) for 30 min. Atomic Layer Deposition (ALD) of Aluminum Oxide. Annealed block polymer templates were subjected to O2 plasma (AV Etcher Vision 320) for 2 s to remove any apparent surface wetting layer. ALD was then performed at 80 °C using trimethylaluminum (TMA) as the metal precursor and distilled water as the oxygen source. The temperature of TMA and water were fixed throughout the deposition at −15 °C and ambient, respectively. The deposition thickness per cycle was calibrated using a bare silicon wafer to obtain a deposition rate of 0.1 nm/cycle. ALD was performed using either 100 or 25 cycles in four steps of N2 (20 s), TMA (12 s), N2 (20 s) and H2O (12 s). Dry Etching. Ar ion mill etching (Intlvac Nanoquest Ion Beam Etch) was used to remove excess layers of Al2O3 and the organic template. Thin films prepared using 100 or 25 cycles of ALD were etched for 3 or 1 min, respectively, under an applied beam voltage of 200 V, beam current of 70 mA and acceleration voltage of 24 V. Reactive ion etching (RIE) using O2 plasma for 30 s ensured complete removal of the organic material. The profile recipe consisted of 50 mTorr of pressure, RF of 50 W and 50 sccm of O2 gas. Small Angle X-ray Scattering (SAXS). SAXS measurements on bulk samples were performed at the Advanced Photon Source (Argonne National Lab, Argonne, IL) at beamlines 12-ID-B and 5ID-D. Bulk samples were annealed in hermetic T-zero DSC pans (TA Instruments) prior to exposure at room temperature. Typical exposure times were of order 1 s. Isotropic 2D scattering patterns were recorded on a Pilatus 2 M area detector or a Rayonix area CCD detector. The sample-to-detector distance was calibrated with a silver behenate standard. Typical experiments utilized a sample-to-detector distance of 8500 mm and an X-ray wavelength of 0.729 Å. 2D data were reduced to intensity as a function of the magnitude of the wave vector q = |q| = 4πsin(θ/2)/λ, where λ is the nominal X-ray wavelength and θ is the scattering angle. Grazing Incidence Small Angle X-ray Scattering (GISAXS). All GISAXS measurements were performed at the G1 station of the Cornell High Energy Synchrotron Source (CHESS). The incidence angle was set above the critical angle of the film under study. The wavelength of incident radiation was 1.0716 Å and the sample-todetector distance was 1869.5 mm. Scattering patterns were collected using a two-dimensional CCD camera with an image size of 1024 by 1024 pixels. AFM and SEM Characterization. PCHE-PLA templates and Al2O3 nanoarrays were characterized by atomic force microscopy (AFM) and scanning electron microscopy (SEM). AFM images were collected at room temperature using a Bruker Nanoscope V Multimode 8 SPM operated in tapping mode. NanoWorld NCSTR AFM probes with a tip radius of
