Fabrication of a Highly Dense Line Patterned Polystyrene Brush on

J. Phys. Chem. C 2010, 114, 11801–11809

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Fabrication of a Highly Dense Line Patterned Polystyrene Brush on Silicon Surfaces Using Very Large Scale Integration Processing Jem-Kun Chen* and Ai-Ling Zhuang Department of Polymer Engineering, National Taiwan UniVersity of Science and Technology, 43, Section 4, Keelung Road, Taipei, 106, Taiwan, Republic of China ReceiVed: April 12, 2010; ReVised Manuscript ReceiVed: June 1, 2010

In this study we used a novel fabrication process, involving a very large scale integration and isotropic oxygen plasma treatment to generate highly dense lines and well-defined patterns of polystyrene (PS) brushes on patterned Si(100) surfaces. We defined trench patterns having a duty ratio of 1:1 on the Si surface using electron beam lithography and then applied isotropic oxygen plasma (IOPT) to treat the bottom of the trenches. The resolution of the line patterns of the PS brushes approached 160 nm, with a duty ratio of 1:1. We established the surface grafting polymerization kinetics of the PS chains on the Si surface by analyzing their thicknesses and number-average molecular weights (Mn). The propagation rate (kp) and active grafting species deactivation rate (kd) had values of 3.3 × 10-2 s-1 M-1 and 7.5 × 10-5 s-1, respectively. The measured thicknesses (ellipsometry) and analyzed values of Mn (gel permeation chromatography) of the corresponding “free” PS fit well to the polymerization kinetics model. In addition, we used friction coefficients to verify the structures of the dense lines of patterned PS brushes after immersion in various solvents. We observed different patterns and friction coefficients for the surfaces presenting dense lines of PS brushes grafted from 160 nm wide trenches for 26 h, after immersion in water and toluene, respectively. Thus, the densely patterned PS brushes on the Si surface exhibited solvent-responsive switching properties. Introduction Fabrication of well-defined chemically patterned surfaces on the micro- and nanoscale is a process at the heart of many modern technologies, including microelectronic devices,1 microelectromechanical systems,2 microfluidic devices,3 photonics,4 sensors,5 microarrays,6 and tissue engineering.7 Historically, chemical patterning has been performed using photolithography, a process developed for and by the microelectronics industry to pattern one specific type of material: photoresists. The most sophisticated integrated circuits (ICs), such as microprocessors and memory chips, are patterned using optical lithography. The performance of semiconductor electronics is coupled to the resolution of the lithographic process; to maintain pace with historic growth rates, the dimensions of many contemporary circuit elements must shrink to below 50 nm. This challenge requires the development of high-resolution imaging materials (resists) that offer precise controls over feature sizes and densities, pattern roughness, and defect concentrations over large areas. Stable polymer brushes can provide excellent mechanical and chemical protection to substrates, alter the electrochemical characteristics of interfaces, and open up new pathways for the functionalization of Si surfaces.8-10 Because they are bonded covalently to the substrates, one particular advantage that polymer brushes have over spin-coated polymer layers is their stability against solvents and harsh conditions (e.g., high temperatures). It has been suggested that block copolymer self-assembly in tandem with optical lithography could satisfy the requirements of new high-resolution imaging materials.11,12 Block copolymers are constructed by linking together two (or more) chemically distinct homopolymer chains; immiscibility between the different * To whom correspondence should be addressed. Tel.: +886-2-27376523. Fax: +886-2-27376544. E-mail: [email protected].

segments drives them to self-assemble into periodic mesophases. The advantage of this approach is that thermodynamics will determine the structure of the block copolymer resist rather than the complex chemistry and exposure statistics associated with optical lithography; therefore, the patterns can be highly uniform over large areas. Incorporating these systems into IC production is potentially very simple. The strategy proposed by Nealey et al. uses optical lithography to define a chemical template that directs the placement of each block copolymer domain with respect to the substrate (conceptually similar to epitaxial crystal growth).13 The self-assembly is speculated to “heal” errors in the chemical template, such as line width variations and pattern roughness.14,15 Recent studies have revealed that these concentrated brushes have structures and properties that are quite different and, in some cases, unpredictable from those of semidilute brushes; most strikingly, concentrated poly(methyl methacrylate) (PMMA) brushes swollen in a good solvent (toluene) can exhibit an equilibrium film thickness as great as 80-90% of the full (contour) length of the graft chains, indicating that the chains are extended to a similarly high degree.16 A surface density of 0.7 chains nm-2 for a PMMA brush, for example, also means that the thickness of the dry film reaches approximately 40% of the full length of the chains, which is much larger than the mean size of the chains in a random-coil (or so-called “mushroom”) conformation. Atom transfer radical polymerization (ATRP) has been applied to surface-initiated graft polymerization, allowing controlled grafting of well-defined polymers from various solid surfaces with dramatically high surface densities.17,18 Surfaceinitiated ATRP (SI-ATRP) is an effective method for the production of biomedically and biotechnologically interesting brushes.19,20 SI-ATRP has been used to prepare “nonfouling” poly[oligo(ethylene glycol) methacrylate] (POEGMA) and poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) brushes

10.1021/jp103258n  2010 American Chemical Society Published on Web 06/21/2010

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that effectively resist protein and cell adhesion.21,22 Advances in biomedicine and biotechnology have led to an increasing demand for materials that can undergo specific biological interactions with their environment and, therefore, the development of novel surface functionalization strategies.23 The use of polymers as building blocks for surface modification allows the preparation of “smart” or responsive surfaces based on conformational changes in the polymer backbones. One attractive strategy involves the application of a thin, biologically inert polymer coating that can subsequently be used as a platform to introduce biologically active molecules. In this paper, we report the use of polymerization to chemically amplify surfaces patterned with a very large scale integration (VLSI) system into highly dense, patterned polystyrene (PS) brushes. We covalently bonded ATRP initiators onto hydroxylated surface patterns, prepared using electron beam lithography and isotropic oxygen plasma treatment (IOPT), and then amplified the system vertically using ATRP. This surface-initiated polymerization process provides an avenue toward the rapid fabrication of highresolution patterns of polymers. The various line PS patterns on the Si surface after immersion in water and toluene possessed distinct brushlike and mushroomlike structures, respectively. We established the surface grafting polymerization kinetics of the PS on the Si surface by analyzing the thickness and numberaverage molecular weight (Mn). We also determined friction coefficients on the highly dense PS brush surfaces after their immersion in water and toluene. Experimental Section Materials. Single-crystal silicon wafers, Si(100), polished on one side (diameter: 6 in), were supplied by Hitachi (Japan) and cut into 1.5 × 1.5 cm samples. The materials used for graft polymerization, 3-aminopropyltriethoxysilane (AS) and 2-bromo2-methylpropionyl bromide (BB), styrene, copper(I) bromide, copper(II) bromide, triethylamine (TA), and 1,1,4,7,7-pentamethyldiethylenetriamine (PMDET) were purchased from Acros Organics. Styrene, PMDET, AS, and BB were purified through vacuum distillation prior to use. All other chemicals and solvents were of reagent grade and purchased from Aldrich Chemical. To remove dust particles and organic contaminants, the Si surfaces were ultrasonically rinsed sequentially with methanol, acetone, and dichloromethane (10 min each) and subsequently dried under vacuum. The Si substrates were immersed in hydrofluoric acid solution (50 wt %) for 5 min at room temperature to remove the silicon oxide film. The hydrofluoric acid-treated substrates were then immersed in a mixture of H2SO4 and H2O2 (2:1, mol %) for 5 min and subsequently rinsed with doubly distilled water (a minimum of five times) to oxidize the Si. Synthesis of Initiator-Modified Si Surface. The basic strategy for the fabrication of the patterned polymer brushes using the VLSI process was adapted from a previous study.24 The fabrication of the patterned styrene brushes using the VLSI process is depicted in Figure 1. The Si wafer was treated with hexamethyldisilazane (HMDS) in a thermal evaporator (Track MK-8) at 90 °C for 30 s to transform the hydroxyl groups on the surface of the wafer into an inert film of Si(CH3)3 groups. The photoresist was spun on the HMDS-treated Si wafer at a thickness of 780 nm. Advanced lithography was then used to pattern the photoresist with an array of trenches having dimensions ranging from 200 nm to 10 µm after development. The sample was then subjected to IOPT to form hydroxyl groups on the HMDS-treated surface. The substrate was placed on the bottom electrode with the Si(100) surface exposed to the glow

Chen and Zhuang discharge at an oxygen pressure of about 5 × 10-3 Torr for a predetermined period of time to form peroxide and hydroxyl species for the subsequent graft polymerization experiment. IOPT causes surfaces to become chemically modified (strongly hydrophilic or polar) only in those areas not covered by the photoresist.1,26 The introduction of these polar groups provided a more wettable surface for the preparation of the self-assembled monolayer (SAM) for graft polymerization. To immobilize the ATRP initiator, the Si substrate treated with HMDS and IOPT was immersed in a 0.5% (w/v) solution of AS in toluene for 2 h at 50 °C. The AS units assembled selectively onto the bare regions of the Si surface after IOPT, reacting with the SiO and SiOO species. The sample was immersed in a solution of BB and TA (both 2%, v/v) in tetrahydrofuran (THF) for 8 h at 2 °C. After completing the reaction, the wafer was placed in a Soxhlet apparatus to remove any nongrafted material. This procedure resulted in a surface patterned with regions of ASBB for ATRP and regions of photoresist. The functionalized Si substrates were removed from the solution, washed with toluene for 15 min to remove any unreacted materials, dried under a stream of N2, and subjected to surface-initiated polymerization reactions. Finally, the surfaces were dried under vacuum and stored under a dry N2 atmosphere. Surface-Initiated Atom Transfer Radical Polymerization. For the preparation of PS brushes on the Si-AS-BB surface, styrene, Cu(I)Br, CuBr2, and PMDET were added to dimethylformamide (DMF). The solution was stirred and degassed with Ar for 15 min at 90 °C. The Si-AS-BB substrate was then added to the solution. After various polymerization times, the wafers were placed in a Soxhlet apparatus to remove any unreacted monomer, catalyst, and nongrafted materials. The remaining photoresist was removed from the HMDS-treated surface by rinsing with solvent, leaving behind the chemically nanopatterned surface. The surfaces were then dried under vacuum at 80 °C for 20 min. The polymer-modified Si surfaces were analyzed using ellipsometry (SOPRA SE-5, France) and X-ray photoelectron spectroscopy (XPS; Scientific Theta Probe, U.K.). In addition, samples of “free” PS were synthesized in solution under the same conditions as those used for grafting polymerization to provide polymers having the same molecular weights of PS as the brushes grafted on the Si surface. The free PS was analyzed using gel permeation chromatography (GPC), performed using a VISCOTEK-DM400 instrument and a LR 40 refractive index detector. Monodisperse polystyrene standards (Polymer Lab, Agilent) were used to generate the calibration curve. The monomer conversion was determined gravimetrically. The resolutions and friction coefficients for various patterns of lines of PS brushes after solvent treatment were measured using atomic force microscopy (AFM; Veeco Dimension 5000 scanning probe microscope) and high-resolution scanning electron microscopy (HR-SEM; JEOL JSM-6500F, Japan). Results and Discussion Immobilization of Initiator on the HMDS- and IOPTTreated Si Surface. To prepare polymer brushes on the Si surface, it was necessary for us to immobilize a uniform and dense layer of initiators on the Si surface. We used XPS to determine the chemical compositions of the pristine Si(100) surface and the Si surfaces at various stages during the surface modification process.25 A single peak component at a binding energy (BE) of about 402 eV, attributable to N-H species, appears in the N 1s core-level spectrum of the Si-AS surface (Figure 2a). Treatment of the Si-AS surface with BB grafted the halogen group for ATRP and yielded the Si-AS-BB surface.

Polystyrene Brush on Silicon Surfaces

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Figure 1. Schematic representation of the process used to fabricate surfaces chemically nanopatterned with PS brushes. (A) Si wafer treated with HMDS in a thermal evaporator. (B) Photoresist spin-coated onto the Si surface presenting Si(CH3)3 groups; advanced lithography used to pattern the photoresist with arrays of trenches and contact holes on the surface. (C) Isotropic oxygen plasma etching used to chemically modify the exposed regions presenting Si(CH3)3 groups and to convert the topographic photoresist pattern into a chemical surface pattern; photoresist removed through treatment with solvent. (D) AS selectively assembled onto bare regions of the Si surface. (E) BB selectively reacted with assembled AS onto bare regions of the Si surface to form the initiator. (F) Sample grafting via surface-initiated ATRP of styrene from the functionalized areas of the patterned SAM.

The disappearance of the signal for the N-H species at 402 eV confirmed that the Si surface was ideally halogen-terminated after treatment with BB. The weak signal for N-C species at a BE of 400 eV confirms that the Si-AS-BB surface was ideally halogen-terminated on the initiator-functionalized Si surface (Figure 2a). No signal appeared in the Br 3d core-level spectrum of the Si-AS surface prior to treatment with BB. Treatment of the Si-AS with BB converted the N-H groups to N-C groups and presented the Si surface with Br species, which appeared as a signal at a BE of 70 eV in the Br 3d core-level spectrum of the Si-AS-BB surface (Figure 2b). The PS brushes were monitored in terms of their C and Br elemental components after various grafting times on the Si-AS-BB surface. A single peak component at a BE of about 284.5 eV, attributable to C-H species, appears in the C 1s core-level spectrum of the Si-ASBB surface. Figure 3 displays the elemental C/Si and Br/Si component ratios determined from the C 1s, Si 2p, and Br 3d core-level spectra of the PS brush surfaces obtained after grafting for various times. The C/Si and Br/Si ratios were determined from the sensitivity factor-corrected C 1s, Br 3d, and Si 2p core-

level peak area ratios obtained at a photoelectron takeoff angle (R) of 75°. The values of the [C]/[Si] and [Br]/[Si] ratios of the Si-AS-BB surface obtained from XPS analysis were 5.5 and 0.85, respectively, in fairly good agreement with the theoretical ratios (7 and 1, respectively). The Br/Si ratio decreased and the C/Si ratio increased after grafting the PS brushes on the SiAS-BB surface. We observed an approximately linear increase in the C/Si ratio of the grafted PS layer on the Si-AS-BB surface upon increasing the polymerization time to 16 h, after which time it reached a plateau, indicating completion of the grafting reaction of the PS brushes. The Br/Si ratio decreased from 0.85 to 0.08 upon increasing the polymerization time to 26 h, consistent with some remaining Br species being hidden within the PS brushes. This observation suggests that the PS brushes became entangled, thereby passivating the reactivity of the halogen groups in toluene, causing the thickness of the grafted polymer brushes formed by ATRP to reach a plateau after a certain time, consistent with previous reports.27,28 Our results imply that the grafting species (halogen group) for ATRP became deactivated upon increasing the polymerization time.

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Figure 4. Schematic depiction of surface-confined ATRP: competitive step growth and termination processes.

Figure 2. XPS core-level spectra of the (a) N 1s and (b) Br 3d energy levels of the Si-AS and Si-AS-BB surfaces, respectively.

Figure 3. Elemental C/Si and Br/Si component ratios determined from XPS C 1s, Si 2p, and Br 3d core-level spectra of PS brush surfaces after grafting for various times.

initiated by the initial number of active grafting sites per unit area, I0, a value that essentially depends on the primer composition. Polymer chains grow by one unit of monomer per step at a propagation rate of kp in a second-order reaction, resulting in a polymer consisting of a chain of i monomers after i steps. We also assumed that, under the conditions employed in our experiments, the active grafting species in each step were susceptible to deactivation (irreversibly) by a first-order reaction operating at a rate kd. For simplicity, we assumed that the rate constants kp and kd were independent of the chain length. We define N as the total number of polymerized monomer units on the surface per unit area, M as the concentration of the monomer in solution, Pi as the number of growing polymer chains of length i per unit surface area, and t as the polymerization time. The symbol P0 represents the time-dependent concentration of active initiators that have not reacted with a monomer on the surface; the value of P0, when t is 0, is I0. Note that M is a constant over the course of the polymerization reaction because the number of monomers that become attached to the surface in the form of the polymer is an extremely small fraction of the total number of monomers in solution. The amount of polymer deposited on the surface was proportional to the amount of monomer consumed. The total rate of monomer consumption was equal to the sum of the rates of monomer consumption in each propagation step; therefore, according to the chemical reactions described in Figure 4, the term dN/dt is defined by n

Characterization of PS Brush Structures on Si Surfaces. To investigate the surface polymerization kinetics, it was necessary for us to take into consideration two important facts related to the mechanism. First, the grafting rate was sensitive to the concentration of the monomer. Second, the grafting species became completely deactivated over a period of several hours. The film growth kinetics could be explained using a set of rate equations based on the coupled chemical reactions presented in Figure 4. Several assumptions underlie this analysis. We assume that the surface-tethered ATRP was

∑

dN Pi ) kpMP0 + kpMP1 + · · · +kpMPn ) kpM dt i)0

(1) The concentrations of the propagating chains Pi are timedependent; their values must be derived from the rate expressions that govern the reactions of each individual chain. For P0, the surface concentration of active initiators that have not reacted with monomer, the relevant differential equation is



dP0 ) -kpMP0 - kdP0 dt

(2)

This equation is easily integrated, subject to the boundary condition that I0 equals P0 when t is 0. An expression for the time dependence of P0 can be calculated:

P0 ) I0e-(Mkp+kd)t

(3)

For the values of Pi for i greater than 0, the change in the concentration of a chain of length i depends on the concentration of the chain of length i - 1; therefore, we developed an expression for the time dependence of Pi-1:

dPi ) kpMPi-1 - (kpM + kd)Pi dt

for

i>1

(4)

Solving these equations required a bootstrapping sequence: by substituting the expression previously found for Pi-1 and then integrating, we found an expression for the time dependence of the next variable, Pi:

P1 ) I0kpMte-(Mkp+kd)t 1 P2 ) I0(kpM)2t2e-(Mkp+kd)t 2 (kpM)i i -(Mkp+kd)t P2 ) I te i! 0

(5)

Substituting the expressions for Pi derived above into eq 1 and integrating the result afford an expression (eq 6) for N, the total amount of polymer per unit area.

[

∫ dN ) kpMI0 ∫ e

-(kpM+kd)t

(kpM)2 2 1 + kpMt + t + 2! 3 (kpM) 3 t + · · · dt (6) 3!

]

Integration and simplification of eq 6 leads to the function shown in eq 7 after applying the boundary condition that N equals 0 when t is 0.

N ) kpMI0

∫

[

e-(kpM+kd) 1 + kpMt + 3

]

(kpM)2 2 t + 2!

(kpM) 3 t + · · · dt ) kpMI0 e-(kpM+kd)tekpMtdt 3! (kpMI0) N) (1 - e-kdt) (7) kd

∫

The value of N is further related to the ellipsometric thickness (d) of the film:

d)

Nm0 kpMI0m0 ) (1 - e-kdt) F kdF

(8)

where F is the density of the polymer and m0 is the mass of the monomer unit. Furthermore, we assume that all of the surfacetethered polymer chains have similar structures on the surface because we were using an ATRP system. We define Ms as the surface-average molecular weight (total molecular weight per initial number of active grafting sites on the surface) on the surface:

Ms )

Nm0A kpM (1 - e-kdt) ) I0A kd

where A is the area of the surface.

(9)

Figure 5. (a) Ellipsometric thicknesses of polymer films recorded as a function of polymerization time at a monomer concentration of 1 M. The plots were fitted to the thicknesses determined from the surface grafting polymerization kinetics [d ) (NM0)/(F) ) (kpMI0m0)/(kdF)(1 - e- kdt)]. (b) Number-average molecular weight (Mn) of measured “free” PS, determined through GPC, plotted as a function of polymerization time at a monomer concentration of 1 M. The plots were fitted to the values of Ms determined from the surface grafting polymerization kinetics [Ms ) (Nm0A)/(I0A) ) (kpM)/(kd)(1 - e- kdt)].

Figure 5a displays the thicknesses of the PS brushes grafted for various times on the Si-AS-BB surfaces under a styrene concentration of 1 M. We considered a polymerization time of 26 h to be “infinite” time for ATRP using 1 M styrene. The thickness reached a plateau under these conditions, indicating deactivation of the grafting species (halogen groups) for ATRP. The data in Figure 5a fit well to eq 8. For a styrene concentration of 1 M, the values of (kpM/kd)(m0I0/F) and kd derived from the fit were 34 nm and 7.5 × 10-5 s-1, respectively. The number of polymer chains bound per square centimeter should be approximately equal to the number of alkylsiloxane initiators per unit area. For linear alkylsiloxanes featuring 10-18 carbon atoms, the area per RSi unit at full coverage is about 2.1 nm2.29 We used a value of 102 g mol-1 for m0 and a value of 1.052 × 10-21 g nm-3 for the density of bulk PS (i.e., the density of the monomer) to approximate the density of the polymer (F). The resulting estimated value of kp was about 3.3 × 10-2 s-1 M-1. On the basis of the assumptions above, we obtained a rough estimate for the unitless term kpM/kd (the effective chain length for M ) 1 mol L-1) of about 442. Furthermore, we obtained the surface-average molecular weights (Ms) of the PS brushes on the Si surfaces from eq 9 for various polymerization times; Figure 5b presents additional data for the number-average

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Figure 6. AFM images of PS brush surfaces obtained after polymerization for (a) 4, (b) 9, (c) 16, (d) 21, and (e) 26 h.

molecular weight for the controlled polymerization of “free” PS formed from the free initiator. The values of Mn of the “free” PS in Figure 5b were fit well by eq 9 for a monomer concentration of 1 M up to a polymerization time of 12 h; beyond this time, however, the number-average molecular weights of the “free” PS were larger than the calculated values. These results suggest that the values of kd for the ATRP performed in solution could be ignored because they were extremely low, presumably due to the surface-tethered polymer brush chain having lower degrees of freedom and, therefore, causing the grafting species to become deactivated as a result of entanglement between the polymer brushes. The linear relationship between ln([M0]/[M]) and time verified the firstorder kinetics for ATRP in solution, where [M0] is the initial monomer concentration and [M] is the monomer concentration. The polydispersity index (PDI, Mw/Mn) of the free PS was about 1.05 for a monomer concentration of 1 M. The grafting species

for ATRP deactivated when the chain length increased, because the coil structure of the polymer impeded the ATRP process. Although we did not determine the exact molecular weight of the polymer grafted on the Si surface, we expected the molecular weight of the grafted polymer to be similar to that of the polymer’s calculated surface-average molecular weight; that is, we predicted the molecular weight of the grafted PS formed in solution to increase linearly with respect to the thickness of the PS brushes up to a polymerization time of 12 h. Because of deactivation of the grafting species, the thicknesses of the PS brushes after 12 h of polymerization reached an approximately constant value that corresponded to the surface-average molecular weight for a monomer concentration of 1 M. We developed a model to estimate the molecular weight of a PS brush on a Si wafer in terms of the values of kp and kd. The grafting density (Ds, chains nm-2) may be calculated according to the equation


Ds ) dFNa /Mn


(10)

where Na is Avogadro’s number (molecules mol-1). We obtained a grafting density of 0.48 chains nm-2 for the PS brushes grafted for 26 h at a monomer concentration of 1 M. Furthermore, we calculated the surface coverage, defined as the amount of grafted polymer per square meter of surface, from the product of the thickness and the density of the grafted polymer layer:

surface coverage ) film thickness × density

(11)

For simplicity, we used the density of the corresponding bulk polymer as the density of the grafted polymer film. Thus, we calculated a surface coverage of 3.6 × 10-6 g cm-2 for the PS brushes obtained after a polymerization time of 24 h at a monomer concentration of 1 M. Surface Topography. We used AFM to visualize the topographies of the Si-AS-BB surface and the PS brushes grafted onto the Si surfaces through ATRP for up to 26 h at a monomer concentration of 1 M (Figure 6). The root-mean-square surface roughness (Ra) of the Si-AS-BB surface was 4.627 nm and those of the PS brushes obtained after polymerization times of 4, 9, 16, 21, and 26 h were 4.1, 3.1, 2.8, 1.6, and 1.1 nm, respectively. These results suggest that the grafted PS brushes during the initial ATRP period formed brushlike regimes, because brushlike regimes of the grafted PS brushes were evident on the surfaces presenting short polymer chains. The tethered PS chains gathered to form thin films when the polymerization time increased, because the long PS brushes featured coil-like regimes on the surfaces, causing the values of Ra of the grafted PS brushes on the surface to decrease accordingly. The final step in our strategy was the surface-initiated polymerization of styrene from the functionalized areas on the patterned SAM. The presence of reactive OH groups after IOPT allowed their direct use in the polymerization step. We used lithography processes with positive photoresists to fabricate trenches and holes with resolutions ranging from 200 nm to 1 µm. The use of lithographically patterned PS brushes allowed us to precisely verify the measured thicknesses of the PS brushes using ellipsometry. Figure 7 displays the SEM image of a trench, patterned using e-beam lithography, having a resolution of 160 nm on the Si wafer. The interval between trenches was 160 nm, providing a so-called duty ratio of 1:1. We treated these dense trenches with IOPT to create OH groups on their bottom surfaces. We then grafted PS brushes from these dense trenches through ATRP for 26 h at a monomer concentration of 1 M. Using AFM to visualize the topographies of the resulting PS brushes, Figure 8a reveal that the styrene polymer chains grafted for 26 h on the Si surface existed as a dense distinctive overlayers having widths of 160 nm over scanning areas of 1 × 1 µm. The dense lines of PS brushes, grafted from the dense trenches, resulted in a regular hill-like structure having a top width of 48.6 nm and a bottom width of 230.4 nm. This observation suggests that the tethered PS on the edge of the pattern have more freedom than the center. This allows them to relax into a more stable conformation and relieves some of the entropic penalty for the brushes, causing a stretch to the bottom. Because the substrate is treated by HMDS, this phenomenon is enhanced by a rich alkyl group on the surface. The intervals between the PS brushes were covered partially by the stretched PS brushes, thereby increasing the bottom width. Figure 8b reveals a cross-sectional profile of the densely patterned PS brushes having line widths of 160 nm. The average height of these dense lines of PS brushes grafted for 26 h from trenches having a resolution of 160 nm was about 23.8 nm. This average height was less than the thickness of the PS brushes

Figure 7. SEM image of the trenches, developed using electron beam lithography, with a resolution of 160 nm.

prepared in the absence of lithographed patterns, because the height of the PS brushes was dependent on the grafting site area.30 In a previous study, the formation of the polymer brushes was examined under richly grafted sites when the lithography pattern had dimensions of greater than about 5 µm.31 To investigate the deformation of PS brushes under the influence of various solvents, we took the sample possessing dense lines of grafted PS brushes (26 h treatment, 160 nm resolution) and immersed it ultrasonically in water for 3 h. The dense line patterns of grafted PS brushes exhibited no obvious patterns after immersion in water (poor solvent; Figure 9a). Figure 9b displays the cross-sectional profile of the densely patterned lines of PS brushes having widths of 160 nm after water immersion. These AFM images confirm the success of using the chemical amplification of patterned OH-functionalized SAMs to form spatially localized polymer brushes. The surface featured coillike structures in the presence of the poor solvent because of isotropic or nematic collapse of the PS brushes. The hill-like structures of the grafted PS brushes having widths of 160 nm were separated into coil-like structures on the surface after treatment with water. The intervals between the grafted PS brushes with widths of 160 nm were almost covered by the stretched PS chains, resulting in featureless dense line patterns. The dense line patterns of grafted PS brushes from the 160 nm wide trenches reappeared after immersing the sample ultrasonically in toluene (a good solvent for PS) for 3 h. Thus, the grafted PS brushes exhibited mushroomlike and brushlike structures as a solvent-responsive property.24,30,31 This behavior was reversible for several cycles of transformation between the brushlike and mushroomlike regimes of PS brushes having a thickness of 23.8 nm. These results confirmed that the shape of the polymer brushes changed after immersion in good and poor solvents. To describe the shapes of the layers, we determined the aspect ratio, defined as the ratio of height to width, from the thickness and width of the grafted polymer layer. The grafted PS brush prepared for 26 h with a duty ratio of 1:1 featured a high aspect ratio of 0.1 on the Si surface. Because the dense lines of PS brushes exhibited different morphologies after solvent treatment, we measured the water contact angles on the PS surfaces. We found, however, similar water contact angles for the densely lined PS brushes obtained after various polymerization times. AFM measurements of mechanical properties are typically performed by moving the probe with respect to the sample surface in either the normal or the lateral direction.

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Figure 8. (a) AFM images of densely patterned lines PS brushes (1:1 duty ratio, 160 nm resolution) obtained after polymerization for 26 h; scanning area: (a) 1 × 1 µm. (b) Cross-sectional profile of the densely patterned lines of PS brushes (160 nm resolution) obtained from the AFM image in (a).

Figure 9. (a) AFM image of densely patterned lines of PS brushes (160 nm resolution) obtained after polymerization for 26 h at a monomer concentration of 1 M and immersion in water. (b) Cross-sectional profile of densely patterned lines of PS brushes (160 nm resolution) obtained from the AFM image in (a).


J. Phys. Chem. C, Vol. 114, No. 27, 2010 11809 during the ATRP process, causing the thickness of the polymer brushes to reach a plateau. In addition, we used friction coefficients, determined through AFM measurements (contact mode), to verify the deformation of the densely patterned lines of PS brushes after immersion in good and poor solvents, respectively. The solvent-responsive properties of these PS brushes featuring dense line patterns, between the mushroomlike and brushlike regimes, suggests their use in the development of switchable high-resolution imaging materials. Acknowledgment. We thank the National Nano Device Laboratory for financially supporting the electron beam lithography system used in this study. References and Notes

Figure 10. Friction coefficients of the densely patterned lines of PS brushes (duty ratio, 1:1) grafted from 160 nm wide trenches for 26 h, recorded after immersion in water and toluene, respectively.

Measurement in the normal direction is usually referred to as force spectroscopy or force distance curve measurement, which can yield adhesive forces between the tip and the sample as well as the elastic modulus and hardness.31 We used the friction coefficients, determined through AFM in contact mode, to confirm the structural transformations occurring after various solvent immersions. Figure 10 displays the friction coefficients of the surfaces presenting densely patterned lines of PS brushes grafted onto 160 nm trenches after immersion in water and toluene. Friction forces in dense line patterns and flat surfaces of PS brushes, as well as the rolling friction forces for particles, have been studied previously.32 We observe approximate linear increases in the friction coefficients of the densely patterned lines of grafted PS layers with respect to the grafting time, after immersion in water and toluene, respectively. These results suggest that, upon increasing the grafting time, the densely patterned lines of PS brushes, grafted from 160 nm wide trenches, featured larger coil-like regimes that entangled the AFM tip, leading to increased friction coefficients. Furthermore, the surfaces presenting densely patterned lines of PS brushes had smaller friction coefficients after immersion in water than they did after immersion in toluene. Direct measurement of the sliding friction between two dense lines of PS brushes was complicated by the fact that the interaction geometry led to different magnitudes of normal and lateral forces for each point during the sliding of one dense line over the other.33 These findings confirm that the dense lines of PS brushes grafted from the 160 nm wide trenches featured various morphologies, resulting in various friction coefficients on the surface after immersion in water and toluene. Conclusions We have used a “grafting from” system with ATRP to prepare well-defined densely patterned lines of PS brushes grafted from 160 nm wide trenches (prepared using electron beam lithography) on Si wafers. This novel strategy allows the fabrication of patterned polymer brushes from Si surfaces using commercial semiconductor processes. The key feature of this approach is the use of surface-initiated polymerization through IOPT to chemically amplify patterned SAMs into macromolecular films. It provided patterned polymeric thin films having surface properties that approached nanodense line patterns having a duty ratio of 1:1. From the thickness and number-average molecular weight, we estimated the surface grafting polymerization kinetics of the PS chains on Si surfaces. Our kinetic model suggested that the grafting species (halogen groups) became deactivated

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JP103258N

Fabrication of a Highly Dense Line Patterned Polystyrene Brush on

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