Density Doubling of Block Copolymer Templated ... - ACS Publications

Letter pubs.acs.org/NanoLett

Density Doubling of Block Copolymer Templated Features Nathanael L. Y. Wu,†,‡ Xiaojiang Zhang,†,§ Jeffrey N. Murphy,†,§ Jinan Chai,†,∥ Kenneth D. Harris,*,† and Jillian M. Buriak*,†,§ †

National Institute for Nanotechnology (NINT), National Research Council, 11421 Saskatchewan Drive, Edmonton, Alberta T6G 2M9, Canada ‡ Department of Electrical and Computer Engineering, University of Alberta, Edmonton, Alberta T6G 2G8, Canada § Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada ∥ Department of Mechanical Engineering, University of Alberta, Edmonton, Alberta T6G 2G8, Canada S Supporting Information *

ABSTRACT: Block copolymers can be used to template large arrays of nanopatterns with periodicities equal to the characteristic spacing of the polymer. Here we demonstrate a technique capitalizing on the multilayered arrangement of cylindrical domains to effectively double the pattern density templated by a given polymer. By controlling the initial thickness of the film and the solvent annealing conditions, it was possible to reproducibly create density doubled lines by swelling the film with solvent until bilayers of horizontal cylinders were obtained. This process was also demonstrated to be compatible with graphoepitaxy. KEYWORDS: Block copolymer, solvent anneal, self-assembly, density doubling, graphoepitaxy, thin films, nanolines

T

he remarkable development of the semiconductor industry over the past decades has largely been driven by the ongoing increase in feature resolution on electronic materials.1 While improvements to projection lithography have continually been implemented to reduce feature critical dimensions, it has become increasingly difficult and expensive to continue this trend due to the inherent diffraction limits of light.2,3 As alternative methods for nanoscale patterning including extreme ultraviolet lithography and nanoimprint lithography have yet to be broadly implemented, interest has grown in “bottom-up” approaches, which utilize the selfassembly of materials such as block copolymers (BCPs), to enable the continued reduction of feature size for semiconductor devices. The International Technology Roadmap for Semiconductors (ITRS) includes BCP patterning as a future pathway to generating high-resolution patterns.1 BCP self-assembly has been the subject of intense research as a result of the ability of these macromolecules to create dense arrays of nanoscale structures with periodicities reaching less than 10 nm through nanoscale phase separation.4−6 The selfassembly process is facilitated by various annealing techniques,4,7−10 which when combined with sparse prepatterned chemical11 or topological12 features can induce the formation of BCP arrays with a high degree of lateral ordering within minutes.9 These regular patterns can then be used as templates in applications such as photonic crystals,13,14 flash memory,15 nanowire transistors,16 and memory storage devices.17−20 Recent works have also demonstrated various techniques using complex prepatterned structures to coax BCP films into forming nonregular, device-oriented structures that are useful for producing the designs required in nanoelectronic devices.21,22 © 2011 American Chemical Society

Here, we demonstrate a simple and efficient method for doubling the density of line structures templated by BCP selfassembly. In annealed BCP films containing two layers of parallel horizontal cylinders, cylinders of the upper layer form over the interstices of the cylinders in the underlying lower layer (Figure 1A) as described in various studies.23−26 While each layer of cylinders has a pitch of L, if the matrix between the cylinders is removed, the three-dimensional assembly of parallel cylinders collapses to form an array of parallel cylinders with half the original pitch (L/2). This effectively increases the characteristic feature density of a given BCP by a factor of 2 without requiring multiple steps of polymer pattern transfer. Cylinder-forming polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) was chosen as the system to demonstrate the bilayer density doubling technique and to further understand the parameters available for controlling the self-assembly. Toluene solutions of the BCP were spin-coated onto clean silicon substrates,27 and film thicknesses were controlled by adjusting the concentration of the BCP solution and the spin speed. The BCP films were then solvent annealed in a chamber containing a 10:1 (v/v) mixture of tetrahydrofuran (THF) and water, which enabled the BCP to self-assemble into parallel horizontal cylindrical structures. The resulting soft patterns were then fixed by a metallization and plasma treatment28,29 to provide improved visualization via scanning electron microscopy (SEM) and atomic force microscopy (AFM) and a potential means for pattern transfer. SEM images of platinum Received: October 5, 2011 Revised: December 5, 2011 Published: December 14, 2011 264

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Figure 1. (A) A schematic of the self-assembled horizontal cylindrical structures in a polymer film. In areas where only a monolayer of cylinders existed, lines with characteristic spacing of L were observed after the metallization step was performed (B). In areas where bilayers of BCP cylinders existed, the line spacing was halved to L/2 (C). The cross-section image (D) demonstrated that both layers of lines were made of clearly identifiable structures, with the top layer lines being slightly brighter and narrower than those from the underlying layer. It was also possible to create double-layer dot patterns (E−G), making it possible to double the densities of dot patterns using this technique. All scale bars are 100 nm.

lines templated by a monolayer of cylinders with a pitch of 47 nm (Figure 1B) and by a bilayer of cylinders with a pitch of 24 nm (Figure 1C)both from PS-b-P2VP with Mn(PS) = 50 kg/ mol and Mn(P2VP) = 16.5 kg/mol [henceforth denoted as PS(50k)-b-P2VP(16.5k) ]demonstrate that the line densities were doubled using this approach. The cross-section SEM in Figure 1D clearly shows that after metallization and plasma the metal lines resulting from the upper and lower layers could be readily distinguished due to their contrast in the SEM. This density doubling approach also has the potential to create structural arrangements not normally accessible using monolayer patterning techniques, such as the doubled hexagonal dot arrays derived from bilayers of hexagonally close-packed spherical domains30 shown in Figure 1F,G. These patterns have been observed as an intermediate phase during the transition from low-order dot patterns to high-order density doubled lines (see Figures S1 and S2) and have been obtained over large areas by annealing PS(125k)-b-P2VP(58.5k) with neat THF. Density doubling should be generalizable for pattern formation in other BCP systems so long as one block can be selectively removed31 (in the present case, the PS block). If inherently etch-resistant polymer blocks such as PDMS8,32 or PFS33 are chosen, or if the resulting density doubled structures are made etch-resistant through further processing steps,28,29,34 then these patterns from alternative BCPs could be used as etch masks in subsequent patterning steps.32 This density doubling method was applied to a range of cylinder-forming PS-b-P2VP polymers of varying molecular weights (Figure 2A−F) . The line patterns in each panel were obtained after 20 h of annealing with the 10:1 mixture of THF and water. Neat THF was also tested; however, the line patterns produced were rougher and contained more discontinuities (see Figure S1). As observed in the SEM images, lines templated by the upper BCP layer were brighter and slightly narrower than those templated by the bottom BCP

Figure 2. Six SEM micrographs of metal line patterns formed using the bilayer approach for various molecular weights of PS-b-P2VP block copolymer: PS(125k)-b-P2VP(58.5k) (A), PS(56k)-b-P2VP(21k) (B), PS(50k)-b-P2VP(16.5k) (C), PS(44k)-b-P2VP(18.5k) (D), PS(32k)b-P2VP(12.5k) (E), and PS(23.6k)-b-P2VP(10.4k) (F). The samples were each annealed with vapor from a 10:1 THF/water mixture for 20 h and subjected to a metallization and plasma step. In (G), an AFM image of density doubled Pt lines templated from the PS(50k)-bP2VP(16.5k) polymer is shown. Height data along the white line are plotted in (H). All scale bars are 100 nm.

layer, possibly due to greater loading of the metal ion precursor in the upper layer. To determine whether the lines were originally part of the upper or lower layer, samples with disordered lines were analyzed: the brighter lines were found to consistently cross over the dimmer lines, suggesting that the brighter lines were from the top layer, while the dimmer lines were from the underlying layer (see Figure S3). A line scan obtained from the AFM (Figure 2G) of PS(50k)-b-P2VP(16.5k) after the metallization and plasma steps confirmed that the lines from the top layer were higher relative to those from the underlying layer by about 10 nm. As summarized in Table 1, monolayer line spacings ranging from 35 to 84 nm using BCPs ranging from PS(23.6k)-b-P2VP(10.4k) to PS(125k)-bP2VP(58.5k) were approximately halved using the density doubling method. For comparison, monolayer line spacings from thermally annealed samples are given together with the spacings from solvent annealed samples in Table 1. Line edge roughness (LER) and line width measurements were also performed on a selection of BCP samples on 265

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Table 1. Line Spacings for Various Polymers after Different Anneal Conditions line pitches and standard deviations (nm) polymer PS(125k)-bP2VP(58.5k) PS(56k)-bP2VP(21k) PS(50k)-bP2VP(16.5k) PS(44k)-bP2VP(18.5k) PS(32k)-bP2VP(12.5k) PS(23.6k)-bP2VP(10.4k)

thermal anneal

solvent anneal monolayer

solvent anneal bilayer

dots

84 ± 2

41 ± 5

dots

50 ± 4

25 ± 1

45 ± 5a

48 ± 3

24 ± 1

49 ± 7

52 ± 1

26 ± 1

40 ± 9

43 ± 4

21 ± 2

30 ± 6

35 ± 4

17 ± 3

a

Mixture of dots and horizontal lines; line spacing from the horizontal lines is shown.

unpatterned silicon annealed in the THF/water mixtures. The LER was evaluated for line segments at least 15 μm long, and 3σ values were measured. The measured LER values ranged from 2.0 nm [PS(32k)-b-P2VP(12.5k) ] to 4.3 nm [PS(50k)-bP2VP(16.5k) ] for the bright lines and from 2.2 nm to 5.4 nm for dark lines from the same samples. The data showed that dark lines templated from the lower BCP layer were somewhat rougher (2% to 44% greater LER values) than lines templated from the upper layer, but all of the measured LER values were found to be similar to LER values in contemporary structures fabricated by electron beam lithography.35 A complete data table is provided in the Supporting Information. To investigate the mechanisms of density doubling, a chamber for solvent annealing (see Figure S5) was designed to accommodate in situ ellipsometry and optical microscopy in order to monitor changes in film thickness throughout the annealing process. PS(50k)-b-P2VP(16.5k) was annealed in the presence of the 10:1 mixture of THF and water for up to 90 min. As shown in Figure 3, during a typical anneal, absorption of solvent by a 41 nm thick BCP film led to swelling that eventually plateaued at ∼116 nm. The evolution of BCP structures as the film swelled was tracked by subjecting samples of identical initial thickness (d0) and composition to the same annealing conditions and removing them when various thicknesses (ds) were attained. To terminate the anneal, the chamber was opened and immediately purged with air, causing the film to relax back to its original thickness. Disordered micelles were observed for the initial as-cast film, but with greater annealing, the templated structures transformed into quasi-hexagonal dot patterns at a degree of swelling (D = ds/d0) around D ≈ 1.6, which were consistently observed until D ≈ 2.5 was reached. As the films continued to swell to D ≈ 2.7, the dots connected with adjacent dots to form short line patterns with periodic bulges, similar to the “necks” observed in previous studies.36 Density doubled line patterns were also present at a few locations but were not prevalent across the entire substrate at this point. After 90 min, and with D ≈ 2.8, density doubled line patterns were observed uniformly across the sample, as shown in Figure S6. The swelling of these (originally 41 nm thick) films was also accompanied by color changes from brown to purple and blue due to thin film interference effects.26,37 These color changes were simultaneously recorded and are displayed as the color gradient in Figure 3.

Figure 3. Time evolution of film thickness throughout a typical 90 min annealing process for a d0 = 41 nm film is shown in the plot, with the color beneath the line matching the visually observed film color at corresponding stages during the anneal. Representative SEM images of metallized structures templated from identical BCP films, swelled to various thicknesses, are found around the periphery. These SEM images depict the typical structures observed and their evolution during the anneal process. All scale bars are 100 nm.

This evolution of structures observed as the film swelled can be attributed to the increased film thickness25 and the plasticization effect afforded by an increase in solvent content.38 The increased film thickness allows the horizontal, cylindrical bilayer structures to become more thermodynamically favorable, while the additional mobility allows the polymer to selfassemble into more stable configurations. Thus, for films of PS(50k)-b-P2VP(16.5k) to obtain density doubled line patterns in these 90 min anneals, a degree of swelling around 2.8 must be attained for films of this particular initial thickness (41 nm) to favor bilayer structure formation. To explore the dependence of structure morphology on the initial BCP film thickness, samples with initial thicknesses of 33 and 52 nm were also annealed with the 10:1 THF and water mixture for up to 90 min and compared to annealed films initially 41 nm thick (Figure 4A−C). One major difference of the thinner 33 nm film was the presence of both undoubled line structures and density doubled line structures on the same sample, suggesting an insufficient amount of polymer for full density doubled line coverage. One example of an interface between a monolayer line and density doubled line region is displayed in the SEM in Figure 4A. (Another example is shown in Figure S7.) AFM images at an interface before metallization and plasma (Figure 4D) show that the density doubled region is ∼14 nm higher than the monolayer region. For films with greater initial thicknesses of 52 nm, it was observed that the BCP structures were never able to advance beyond the dot phases within 90 min. These observations indicate that the initial thickness of a film affects the ability for density doubled lines to be formed. Thus, for density doubled lines to be produced in a homogeneous fashion over the entire surface, there exists an ideal initial thickness of the BCP film around 41 nm for the BCPs studied here. One key criterion for BCPs to be an industrially viable patterning technique is precise control over the orientation and 266

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initial film thickness and swelling conditions is critical for obtaining uniform features across the sample. Furthermore, the orientation and position of these features can be controlled through the use of graphoepitaxy. This technique holds potential for pushing the limits of feature density for BCP patterning.

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ASSOCIATED CONTENT

S Supporting Information *

Scanning electron micrographs, scanning Auger maps and line scans, photographs of anneal chamber, experimental procedures, and image processing details. 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] (J.M.B.); [email protected]. ca (K.D.H.).

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Figure 4. (A−C) SEM images of metallized structures templated from different initial BCP film thicknesses after annealing for 90 min with the 10:1 THF/water mixture. (D) 5 μm × 5 μm AFM image of a terrace edge in a d0 = 33 nm film after the anneal but before the metallization process. A line scan (position indicated by the white line) shows that the step height is ∼14 nm (E). All scale bars are 500 nm.

ACKNOWLEDGMENTS This work was supported by the NRC-NINT, the University of Alberta, Alberta Innovates Technology Futures, the Canadian Foundation for Innovation (CFI), and NSERC. The authors thank the University of Alberta NanoFab for both clean room and e-beam lithography support and the Alberta Centre for Surface Engineering and Science (ACSES) for their help with scanning Auger microscopy.

position of the self-assembled patterns. Two methods for attaining precise control over these self-assembled structures include graphoepitaxy, which uses prepatterned topographical features, and chemical epitaxy, which is based upon prepatterned chemical surface features to guide the selfassembly of BCP structures.2,3,39 To demonstrate the compatibility of the density doubling technique with graphoepitaxy, SiO2 walls were patterned on the sample substrates by writing lines on a film of hydrogen silsesquioxane (HSQ) resist using electron beam lithography. A subsequent development step in a 25% (w/w) solution of tetramethylammonium hydroxide (TMAH) removed the unexposed HSQ, leaving a silicon wafer with SiO2 lines. A film of PS(50k)-b-P2VP(16.5k) was then spin-coated, annealed, and metallized as before. The resulting structures shown in Figure 5 demonstrate that the

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