Molecularly Functionalized Silicon Substrates for Orientation Control

Jan 30, 2013 - Materials Chemistry Section, Department of Chemistry, University College ... Department of Chemistry, Selcuk University, Konya 42075, T...
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Molecularly Functionalized Silicon Substrates for Orientation Control of the Microphase Separation of PS‑b‑PMMA and PS‑b‑PDMS Block Copolymer Systems Dipu Borah,†,‡,§ Mustafa Ozmen,∥ Sozaraj Rasappa,†,‡ Matthew T. Shaw,†,‡,§,⊥ Justin D. Holmes,†,‡,§ and Michael A. Morris*,†,‡,§ †

Materials Chemistry Section, Department of Chemistry, University College Cork, College Road, Cork, Ireland Centre for Adaptive Nanostructures and Nanodevices (CRANN), Trinity College Dublin, College Green, Dublin 2, Ireland § Tyndall National Institute, Lee Maltings, Prospect Row, Cork, Ireland ∥ Department of Chemistry, Selcuk University, Konya 42075, Turkey ⊥ Intel Ireland Limited, Collinstown Industrial Estate, Leixlip, Co. Kildare, Ireland ‡

ABSTRACT: The use of block copolymer (BCP) thin films to generate nanostructured surfaces for device and other applications requires precise control of interfacial energies to achieve the desired domain orientation. Usually, the surface chemistry is engineered through the use of homo- or random copolymer brushes grown or attached to the surface. Herein, we demonstrate a facile, rapid, and tunable approach to surface functionalization using a molecular approach based on ethylene glycol attachment to the surface. The effectiveness of the molecular approach is demonstrated for the microphase separation of PS-b-PMMA and PS-b-PDMS BCPs in thin films and the development of nanoscale features at the substrate.

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INTRODUCTION Diblock copolymer self-assembly at substrate surfaces can enable the generation of nanoscale structures in a parallel, scalable, bottom-up fashion.1 Thin films of nonsymmetric poly(styrene-b-methylmethacrylate) (PS-b-PMMA),2−5 poly(styrene-dimethylsiloxane) (PS-b-PDMS),6−9 poly(styrene-bethyleneoxide) (PS-b-PEO),10−12 and poly(styrene-b-lactide) (PS-b-PLA)13,14 block copolymers (BCPs) on silicon substrates can be used to generate nanostructures with cylindrical PMMA/PDMS/PEO/PLA microdomains selectively aligned perpendicular or parallel to the surface plane. By selective removal of one of the blocks by dry or wet etching, surface topographical arrangements of feature size < 100 nm can be created.2−5,11−13 These nanostructures have many more applications in the fabrication of semiconductor capacitors,15 metallic,4,12 and magnetic2,3,11,16,17 nanostructures. However, the generation of these structures requires precise control of the substrate−polymer interfacial energies to ensure that the cylindrical microdomains are correctly orientated to the substrate surface plane.18−24 “Neutral” surfaces, that is, surfaces exhibiting equal interaction (wetting) energies with both blocks, will favor a vertical arrangement of cylinders while non-neutral surfaces will favor horizontal orientation.25−28 Note that orientation is also dependent on film thickness, gas− polymer interfaces, surface free-energies, and so forth.29 The appropriate surface treatment also facilitates self-assembly, long-range domain ordering, defect minimization and improves wetting property of the BCP. © XXXX American Chemical Society

One of the most successful forms of surface modification to define the structural orientation of BCPs is the use of covalently anchored hydroxyl-terminated homopolymers or random copolymers, and for the PS-b-PMMA system HO-PS-rPMMA is widely used as a “neutral brush”.26,27,30−32 While this approach is effective and allows fine-tuning of interfacial energies, it also involves lengthy processing time, starting materials that are limited in commercially availability and expensive, as well as yielding a relatively thick underlayer that can interfere with subsequent surface processing to yield the desired substrate structure (see below). BCPs containing inorganic components, for example, PDMS, are particularly useful because they can be processed to directly yield an oxide nanostructure without any selective polymer chemistry.6−9,33−37 PS-b-PDMS has particular relevance because of its high Flory−Huggins parameter (χ ∼ 0.26)38 which allows delivery of sub-10 nm feature size structures. The parallel alignment of PDMS cylinders in the PS-b-PDMS system has been achieved by anchoring with a hydroxyl-terminated PDMS homopolymer.7 However, this does result in the formation of oxide underlayers during processing.6,7 Literature survey reveals that modified alkyl chlorosilane (ACS) self-assembled monoloayers (SAMs) have been used to control the microdomain orientation of PS-b-PMMA BCP thin films on silicon substrates.39−41 The surface energy of a dense Received: October 18, 2012 Revised: January 30, 2013

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Coating Systems, Inc.) onto silicon wafers at 3000 rpm for 30 s. Samples were annealed in a vacuum oven (Townson & Mercer EV018) at 443 K under vacuum (−100 kPa) for 6 h. This allows the end-functional hydroxyl groups of the random copolymers to diffuse to the substrate and react with the silicon native oxide layer, resulting in polymer chain brushes anchored on the substrate. Unbound polymers were removed by sonication (Cole-Palmer 8891 sonicator) and rinsing in toluene. Deposition of EG on Substrate Surface. Solutions of ethylene glycol 1−5% (v/v) were prepared in ethanol and were stirred at room temperature (∼288 K) for 2 h to ensure complete mixing. The hydroxylated substrates (by piranha solution as mentioned above) were immediately coated with ethylene glycol by spin-coating at 1000 rpm for 30 s. Samples were air-dried at room temperature (∼288 K) for 30 min, washed with absolute ethanol and then dried under a stream of nitrogen. Films survived repeated rinses in ethanol as observed by XPS and FTIR. Deposition of Diblock Copolymer and Thin Film Preparation. Block copolymer thin films were deposited onto polymer brush anchored and EG modified planar and channel substrates by spin coating (3200 rpm and 30 s) from dilute solution (1.0−3.0 wt %) of the diblock copolymers in toluene. PS-b-PMMA thin films were thermal annealed in a vacuum oven (Townson & Mercer EV018) at 453 K under vacuum (−100 kPa), well above the glass transition temperatures (Tg) of both PS (373 K) and PMMA (388 K) for 4 h to induce phase separation into hexagonal arrangements of PMMA cylinders in a PS matrix with natural domain period L0 ∼ 36 nm. Samples were removed from the oven immediately after annealing, without any cooling ramp rate. Thin films of PS-b-PDMS were solvent annealed in glass jars under saturated toluene environment at room temperature (∼288 K) for 3 h. Plasma Etching of Block Copolymer Thin Films. The thermally annealed PS-b-PMMA films were subjected to selective PMMA inductively coupled plasma (ICP) etch using an Ar/O2 etch recipe at 1.3 Pa and 100 W with O2 (15 sccm) and Ar (5 sccm) for 6 s to generate a topographical PS pattern in an OIPT Plasmalab System100 ICP180 etch tool. The solvent annealed PS-b-PDMS films were first treated with a CF4 (15 sccm) plasma for 10 s to remove the PDMS surface layer followed by an O2 plasma at 2.0 Pa and 2000 W with 30 sccm O2 for 35 s to partially remove the PS matrix. This process leaves the PDMS cylinders in oxidized from on the substrate. Details are given elsewhere.6,30,32 A schematic showing the deposition of EG onto silicon substrate for BCP self-assembly and plasma etching to develop nanoscale pattern on substrate surface is presented in Scheme 1. Characterization of Materials. Contact Angle. Advancing contact angles (θa) of deionized water was measured on the EG monolayers and polymer brush anchored surfaces at ambient temperature using a Data Physics Contact Angle (model: OCA15) goniometer. Contact angles were measured on the opposite edges of at least three drops and averaged. The values were reproducible to within 1.3°. Film Thickness. Diblock copolymer thin film thickness was determined by ellipsometry (Plasmos SD2000 Ellipsometer). An average of three readings collected from different locations on a sample surface was reported as the film thickness result. Atomic Force Microscopy (AFM). The atomic force microscope (DME 2452 DualScope Scanner DS AFM) was operated in AC (tapping) mode under ambient conditions using silicon microcantilever probe tips with a force constant of 60 000 N m−1 and a scanning force of 0.11 nN. Topographic and phase images were recorded simultaneously. FTIR (Fourier Transform Infrared) Spectroscopy. An IR660 Varian infrared spectrometer was used to record the FTIR spectra. The measurements were performed in the spectral range of 4000−500 cm−1, with a resolution of 4 cm−1 and data averaged over 32 scans. X-ray Photoelectron Spectroscopy (XPS). XPS was performed on Vacuum Science Workshop CLASS100 high performance hemispherical analyzer using an Al Kα (hν = 1486.6 eV) mono X-ray source. Spectra were obtained at a takeoff angle of 15°. Samples were

ACS self-assembled monolayer (SAM) can be adjusted by controlled oxidation sponsored by X-rays41 or UV/ozone treatment.39 Poly(ethylene glycol) has been frequently used as a means to functionalize substrates for controlling surface chemistry.42−44 However, the use of simple, small molecule systems for domain orientation of BCPs has yet to be reported to our knowledge. In this paper, a novel, simple, and rapid method of surface functionalization using small molecules is reported, and demonstration of orientation control is affected for cylindrical phase forming PS-b-PMMA and PS-b-PDMS BCP systems. In this approach, ethylene glycol (EG) was deposited by spin-coating under ambient conditions. The effect of EG was compared to the standard polymer brush systems usually used for these BCPs. The effectiveness of the approach was shown by demonstrating that PS-b-PMMA and PS-bPDMS nanopatterns can be used to create topographical patterns at the substrate surface.

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EXPERIMENTAL SECTION

Materials. The planar substrates used were silicon ⟨100⟩ wafers (ptype) with a native oxide layer of ∼2 nm. No attempt was taken to remove the native oxide of a few nanometers depth. The topographically patterned substrates were etched silicon ⟨100⟩ and processed by means of conventional photolithography, mask, and etch techniques (fabricated via 193 nm UV-lithography). The width and depth of the channel were 280 and 50 nm, respectively. Hydroxylterminated random copolymer composed of styrene (S) and methyl methacrylate (MMA), denoted as HO-PS-r-PMMA, hydroxylterminated PDMS homopolymer, cylinder-forming PS-b-PMMA, and PS-b-PDMS diblock copolymers were purchased from Polymer Source, Inc., Canada and used as received. Detailed characteristics of the polymers are summarized in Table 1. Ethylene glycol (EG,

Table 1. Details of Polymer Characteristics Used for the Present Study molecular weight, Mn, g/mol

polydispersity, Mw/Mn

mole fraction of PS, mol %

volume fraction of PS/PDMS, ϕPS/PDMS

12 400

1.25

0.58

5000

1.07

67 100

1.09

0.68

0.71

45 500

1.15

0.60

0.34

description hydroxylterminated PS-r-PMMA hydroxylterminated PDMS cylindrical PS-b-PMMA cylindrical PS-b-PDMS

CH2(OH)CH2(OH), 95.0%), toluene (99.8%, anhydrous), ethanol (dehydrated, 200 proof), acetone (99.0%, anhydrous), isopropanol (IPA) (99.0%, anhydrous), sulfuric acid (98.0%), and hydrogen peroxide (30.0%) were purchased from Sigma-Aldrich and used without further purification unless otherwise stated. Deionized (DI) water was used wherever necessary. Preparation of Brush Anchored Substrates. Substrates were cut into 2.0 cm2 pieces and then degreased by ultrasonication in acetone and IPA solutions for 5 min each, dried in flowing N2 gas and baked for 2 min at 393 K in an ambient atmosphere to remove any residual IPA. This was followed by cleaning in a piranha solution (1:3 v/v 30% H2O2/H2SO4) (CAUTION! May cause explosion in contact with organic material!) at 363 K for 60 min, rinsed with DI water (resistivity ≥ 18 MΩ/cm) several times, acetone, ethanol, and dried under N2 flow. Piranha activation removes any organic contaminant, greases and creates hydroxyl groups on the silicon substrates. Hydroxyl-terminated PS-r-PMMA and PDMS brush solutions of 1.0 wt % in toluene were spin-coated (P6700 Series Spin-coater, Specialty B

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Scheme 1. Details of the Process Flow of BCP Self-Assembly on Silicon Substrates Functionalized with EG and Subsequent Plasma Etching to Fabrication Nanoscale Features

Figure 1. 2-D tapping mode AFM topography images of EG functionalized silicon substrates deposited from (a) 1%, (b) 3%, and (c) 5% of EG solutions in ethanol by spin coating.

Table 2. Film Thickness and Water Contact Angle of Polymer Brush Layer, EG SAMs and Diblock Copolymer Thin Films on Silicon Substrates film type

EG or polymer concentration, wt %

Si substrate Si substrate HO-PS-r-PMMA brush layer

1.0

PDMS−OH brush layer EG layer EG layer EG layer PS-b-PMMA PS-b-PMMA PS-b-PMMA PS-b-PDMS

1.0 1.0 3.0 5.0 1.0 2.0 3.0 1.0

conditions of film preparation as-received piranha cleaned annealed and cleaned annealed and cleaned annealed and cleaned air-dried air-dried air-dried as-cast as-cast as-cast as-cast

loaded into the vacuum chamber within 1 h after being prepared and were subjected to XPS analysis. Photoemission peak positions were corrected to C 1s at a binding energy of 284.8 eV. Scanning Electron Microscopy (SEM). Top-down and crosssectional SEM images of etched polymer films were obtained by using a high resolution (90% surface area is covered) and in-plane PDMS cylinders are formed as revealed by the plasma etch process. However, film thickness variation is seen at the 1 and 3% EG solution concentrations. This is apparent in both top-down images (as obvious regions where bilayers of cylinders can be seen) and cross-sectional images (considerable irregularity of the cross section). At 5%, there was no observation of multilayer stacking and the cross section

shows a highly regular structure suggesting the surface performs equally as well as the PDMS−OH based brush. The reason for the effective surface chemistry tuning is not due to matching of the hydrophilic/hydrophobic properties. It appears that a PDMS wetting layer is still formed in this process, and it is suggested that the PDMS favoring surface chemistry is due to interactions of the reactive silane and siloxane groups with the EG hydroxyl groups. There is no obvious change in the mean PDMS cylinder spacing, L0, and line width, ⟨d⟩, was found to be similar to that of PDMS−OH brush. Directed Self-Assembly of Block Copolymers by Graphoepitaxy. Further information on the brush and its effects on interfacial chemistry can be gained by carrying out studies on topographically patterned substrates. This allows direct inspection of the wetting of the substrate material at the sidewall. This also shows the capability of the brush to combine with the topography to “direct” the self-assembly process. Illustrative data is shown in Figure 9. For the PS-b-PMMA system, the samples were studied by SEM after selective PMMA etch (Figure 9) and brush and EG coated substrates were performed. As can be seen, within the topography, vertical cylindrical orientation is observed. However, no alignment can be seen (i.e., an alignment of the structure with the topography). This lack of alignment suggests that neither block is preferred at the sidewall, and this is confirmed because there are voids (i.e., PMMA) and PS components at the sidewall. These data thus confirm the “neutrality” of the EG functionalized surface and also show its equivalence in function to the HO-PS-r-PMMA brush. In comparison to the PS-b-PMMA system, the functionalized topography is effective in bringing about directed self-assembly (DSA) of the PS-b-PDMS system. This is because the interface strongly favors the PDMS component. As can be seen from the J

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SEM images of the etched PS-b-PDMS films, EG can produce similar patterns and structures achieved by the PDMS−OH type brush (Figure 10). The horizontal orientation is maintained, and the cylinders tend to align to the topography and the channel sidewall. However, what can be seen is that significantly improved alignment is achieved for the 5% EG coating solution formed functionalized surfaces compared to those at lower concentrations. This is consistent with the improved film quality (simple monolayer films) described above and suggests that the increased EG coverage is needed to optimize the surface chemistry. There may be small differences in defect density of the BCP films between the 5% EG system and the PDMS−OH brush surface, but this is rather difficult to quantify. For both systems, it is most likely that the majority of the defects are “precipitated” by irregular sidewall profiles. These cause variations in channel width and the number of cylinders required to fill the channel, and this results in the type of disclinations and dislocations observed here.

ACKNOWLEDGMENTS Financial support for this work is provided by the EU FP7 NMP project, LAMAND (Grant Number 245565) project, and the Science Foundation Ireland (Grant Number 09/IN.1/602) and is gratefully acknowledged.

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REFERENCES

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CONCLUSIONS We demonstrate here a facile, rapid, and simple method of functionalizing silicon substrates with EG as an alternative to standard polymer brushes for BCP self-assembly to generate nanostructures. Detailed characterization of EG coated surfaces reveals the attachment through hydrogen bonding in particular. The effectiveness of the approach is demonstrated for two cylinder-forming PS-b-PMMA and PS-b-PDMS BCPs. The method showed promise in guiding self-assembly in topographically patterned substrates. It should be noted that the ability to modify surfaces controllably with EG is of considerable importance. The advantage of using a molecular system compared to an expensive, difficult to synthesize random copolymer is obvious. As shown here, by changing coverage, the surface can be modified to show a range of hydrophobic/hydrophilic properties. Recent results have shown that the methodology outlined can be extended to systems such as PS-b-PDMS, PS-b-PEO, and PS-b-P4VP, and this work will be published in due course. The effect of EG on the orientation of PS-b-PMMA and PSb-PDMS is not easily rationalized from simple consideration of surface hydrophilicity or miscibility of the molecule and individual blocks. For PS-b-PDMS, we believe bonding of the PDMS block with hydroxyl groups leads to stronger interactions than might be expected. For the PS-b-PMMA system, we believe the EG acts as a very small surfactant molecule where the ethyl headgroup can interact with the PS block and the hydroxyl tail with the more hydrophilic PMMA block. It is hard to prove such mechanisms further because studies of the molecular confirmation below the BCP film are not possible.

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AUTHOR INFORMATION

Corresponding Author

*Email: [email protected]. Phone: +353 21 4902180. Fax: +353 21 427 4097. Author Contributions

All authors contributed equally to this work. Notes

The authors declare no competing financial interest. K

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