Thin films formed from microphase-separated block copolymers are of interest because they form periodic structures on the order of tens of nanometers with domain size tunable by their molecular weight.[1, 2] This makes them ideal candidates for templates for nanopattering applications, which can be easily integrated into the existing manufacturing processes. Cylinder forming block copolymers have been traditionally used as templates for making parallel stripes in nanopattering applications, like templates for nanowires and polarizers. However, lamellae oriented perpendicular to the substrate have advantages in pattern transfer over parallel cylinders, because of the higher aspect ratio of the resulting templates and vertical side walls. One of the ways to achieve perpendicular orientation of lamellar block copolymer domains is by using brush layers of random copolymers.[6-8] Chemically grafted random copolymers of the brush layers have the effect of modifying the chemical composition of the substrate and in turn tuning the interfacial or wetting behavior of the blocks of the copolymer film on the substrate.
Once the out-of-plane orientation of the block copolymer domains has been controlled, it is important to control their in-plane order, to make then useful for the intended applications. Previously our group has used mechanical shear as an easy alternative approach to orient spherical and cylindrical nanodomains over macroscopic areas.[9-12] Alignment can be achieved with a few minutes of shearing, over cm2 regions of film, and with stresses (order 1000 Pa) small enough that they can be imparted by either a moving solid surface in contact with the film[9, 10] or simply a fluid flowing over the film’s surface.[11, 12]
In this talk we will present recent results in which we have used shear to achieve long range order in a block copolymer thin film forming perpendicular lamellae. Lamellar polystyrene/ poly (methyl methacrylate) (PS/PMMA) block copolymers were made to orient perpendicular to Si substrate by grafting a layer of random terpolymer of poly (styrene/ (methyl methacrylate)/ 2-hydroxyethylmethactylate). To date, films less than one domain spacing thick show good alignment under shear, while thicker films switch back to parallel orientation. Further, we are investigating alignment as a function of stress, terpolymer thickness, terpolymer composition and domain spacing.
Terpolymers of styrene, methyl methacrylate, and 2-hydroxyethylmethacrylate were synthesized using classical free radical polymerization and AIBN initiator at 70⁰C, to a conversion of 20% to avoid compositional drift. Terpolymers with styrene content, Fs, varying from 0.5-0.8 were prepared and the HEMA content in the terpolymers was fixed at 0.01 mole fraction. 1H NMR was used to determine the exact styrene fraction in each of the synthesized terpolymers. The PS/PMMA block copolymer used in this study was purchased from Polymer Laboratories Inc. and had block molecular weights of 28.5 kg/mol PS and 30 kg/mol PMMA. This PS/PMMA block copolymer has a lamellar repeat spacing of ~30 nm as determined from AFM images of thin films.
The brush layer was prepared by spinning a film of terpolymer on bare pieces of 3” silicon wafer from Silicon Quest International and annealing them at 140 °C for 12 hours. Unreacted terpolymer was removed by rinsing with toluene leaving behind a uniform brush layer of 4-6 nm. Dilute solutions (1-2 % in toluene) of the PS/PMMA block copolymer were then spin-coated on the modified Si substrate, at various spin speeds to obtain a range of film thicknesses, and annealed at 150-170 °C to induce phase separation.
Shear stress was applied to the film via PDMS pads (1 cm x 1 cm), prepared by curing a mixture of Dow Corning Sylgard 184/curing agent (10/1 w/w) at 60 °C for 2 h. While holding the film at a constant temperature of 150 °C via a digital hot plate, shear stresses of 10-40 kPa were applied for 30 min. The specimen was then cooled to below the glass transition temperature of both PS and PMMA (~ 100 °C), while maintaining the shear stress, so as to ‘‘lock in’’ the structure induced by shear. The block copolymer domains were then imaged using tapping mode AFM, to determine their orientation.
Results and Discussion.
Use of terpolymer brushes with a range of styrene content enabled the determination of a window of neutrality in styrene mole fraction, which reliably resulted in perpendicularly oriented block copolymer lamellae. As far as we know, the window of neutrality for the lamellae forming PS/PMMA block copolymer lies between a styrene fraction of 0.68-0.74 in the brush layer. Terpolymers with higher styrene fractions (up to 0.8) are still being investigated to determine more accurately this neutrality window. It is to be noted here that this window of neutrality is much higher in styrene content (~ 0.1 mole fraction higher) than that determined by Han et al. . Han et al. prepared their terpolymers using a living radical polymerization, while we use conventional free radical polymerization. The difference in synthesis techniques can result in differences in the detailed chain architecture, resulting in a substantial shift of the window of neutrality. However, our terpolymers (just like those prepared by Han et al.) result in perpendicular lamellae to film thickness up to 2 times the domain spacing of the block copolymer.
Perpendicular domains of block copolymer films of thickness less than 1 domain (< 30 nm) spacing can be shear aligned using stresses of 30 kPa and higher. Stresses lower than 30 kPa do not result in significant alignment of the perpendicular lamellae. This suggests the occurrence of a critical stress for alignment to occur, similar to that seen in sphere and cylinder forming systems. However, shearing films with thickness greater than 1 domain spacing (> 30 nm) results in the lamellae switching back to parallel orientation. There are several possible reasons for this. First, in the current study we have used a brush layer with Fs = 0.68, which is at the edge of the neutral window for forming perpendicular lamellae. It is possible that the substrate is still PMMA preferential, such that mobilization of chains during shear can flip the lamellar orientation. A second possibility is that the film/ PDMS wetting conditions are PS selective (since PDMS has lower surface tension), resulting in the PS domain preferentially wetting the PDMS surface. To overcome these issues we are currently trying to better investigate the window of neutrality in this system, so that we can achieve truly neutral substrates. Also we are working towards neutralizing the PDMS pad used for shearing.
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